Questions about how to get better mileage. [Archive]

CookingWithGas

10-27-2005, 09:45 PM

Also to keep the tires slightly over-inflated (if my tires are rated for 35psi, what is a reasonable amount to over inflate them to?). . . . is 25mph more efficient than 35 or 40? Is it true that you burn up a lot of gas when accelerating?I wouldn't overinflate the tires, you might just be trading one issue for another. You'll get uneven tire wear and go through your tires faster. A set of tires for my van cost around the equivalent of 150 gallons of gas. Do make sure they're fully inflated.

The most efficient point for an engine to operate is at the point of maximum torque, where each stroke of the engine produces the greatest power. When you rev the engine higher than that, you are getting less power per stroke but because you are putting out more strokes you get more power (up to a point where you get the max horsepower). Probably at whatever speed you get at the point of max torque in the highest gear is the most fuel efficient. (I am guessing that speed is still low enough that the engine efficiency trumps drag.) What that is for a given car I can't tell you.

http://www.flamesonmytank.co.za/Articles/torque.htm
http://www.canadiandriver.com/articles/jk/020327.htm

You burn up a lot of gas when accelerating needlessly fast, as has already been said.

Be conscious of how often you brake. If you are braking a lot, it means that you are wasting gas. Driving at 35 mph until you're 10 feet from a stop sign then slamming on the brakes uses more gas than if you coast (i.e., foot off gas--I don't suggest you take it out of gear) and slow down for a couple of hundred yards first. This can be problematic if there is traffic behind you, however. Similarly, in highway driving, leaving a long following distance allows you to make speed adjustments according to the traffic in front without excessive braking. If you're right on someone's bumper in heavy traffic you're going to be going gas-brake-gas-brake constantly.

Balthisar

10-27-2005, 09:50 PM

chaparralv8

10-27-2005, 09:51 PM

I posted this in another thread, but here's my explanation of how the mileage doesn't vary much with speed on the highway:

"Out on the highway, weight becomes slightly less important; aerodynamics and gearing play starring roles here. The slower the engine has to turn on the highway, the better. "Longer" gearing reduces the engine's pumping and frictional losses of the engine. This is why manual-transmission Corvettes get more than 35 MPG just cruising down the highway; that big motor's barely turning above idle. How much your mileage depends on speed generally depends on how aerodynamically slick it is. There are SUVs that burn twice as much gas going 75 as they do going 55, but my old CRX gets the same gas mileage doing 85 as it does going 55 **. The EPA numbers are effectively worthless as the EPA highway test never goes over 48 MPH - if you put a Corvette into 6th at 48 the engine would be shaking like a leaf as the computer tried to prevent it from stalling. "

"**As an engineer, it distresses me somewhat to write this. There are a lot of logical reasons for a car's mileage to drop off at higher speeds: the aerodynamic drag force goes up with the square of the speed, the frictional drag force goes up linearly with speed, and the engine has to turn faster so its pumping losses increase.
However, I like empirical data - it's really what matters at the end of the day. There is one huge reason an aerodynamically slick car gets virtually identical real-world highway mileage over a wide range of speeds. You don't have as wide a speed variation over hills at high speeds - on an uphill grade where you have to floor it to keep the car going 55 in high gear or downshift, you might not even have to open the throttle any more to climb the same hill at 85. Throttle and speed variations are even worse for mileage than just a steady fast pace.
In addition, automakers program injection computers to 1) ace EPA mileage and emissions tests and 2) keep the engine from being damaged during full-throttle abuse by car magazines. The throttle is never floored during an EPA test. As a consequence, there is absolutely no official penalty for a car whose computer tells the injectors to just dump fuel into the engine under full throttle in order to avoid detonation. When journalists fill the tank with the rattiest 87 octane fuel they can find, go out to a SoCal track on a 100-degree day, and pound on the car for hours on end, the last thing the automakers want to hear is that the magazine blew the car up. As a consequence, the mixture is made a lot richer at full throttle than it needs to be - extra fuel decreases combustion temperatures and evaporates on the intake stroke, taking away some more heat when it does.

This means that when you open the throttle past 3/4 of the way to climb a hill on the highway, the car's computer switches to a mode where it just dumps fuel into the engine. If you're moving as many gallons per hour as the injectors will flow, and are'nt going fast, you will not get many miles per gallon"

Mr. Slant

10-27-2005, 10:24 PM

I've recently been playing with my car's new Scangauge.
I've determined that over the course of three week's observations, my car's mileage on my drive to work:
1) Goes up by 3 MPG when I lower my maximum speed from 74 to 55 MPH.
2) Goes up by another 1-2 MPG when I lower my maximum speed to 50 MPH.
3) Gets better (haven't established a reliable numerical metric yet) when I cap my speed at 25 MPH on urban streets on the way to the highway.

And don't worry, I only cap my speed this artificially low when traffic is extremely light or nonexistent.

The vehicle in question is a Mercury Sable with EPA fuel economy stats available here:
http://www.fueleconomy.gov/feg/2001cartablef.jsp?id=17721

Prior to my experiment beginning my median fuel economy was 24.2 MPG on a per-tank basis. I can now get 29.5 MPG (round trip) on my 13 mile 70/30% highway/city trip to work.
For some reason my trips to work run 28 MPG at 50 MPH while they run 31 MPG on the way home at the same top speed. I assume there's a slight grade involved somewhere.

jasonh300

10-27-2005, 10:30 PM

I've recently been playing with my car's new Scangauge.

What is a Scangauge?

Una Persson

10-27-2005, 10:37 PM

The most efficient point for an engine to operate is at the point of maximum torque, where each stroke of the engine produces the greatest power.
I'm sorry, but this is not true as written. Since maximum torque is measured at WOT, you cannot draw a conclusion that the engine will be as efficient at that same engine speed under normal cruising conditions. I've spent more hours than I care to re-live dyno-testing engines to create maps of power versus torque versus brake specific fuel consumption. It is more correct to say that in general terms, under normal motoring conditions where one is trying to minimize fuel consumption, better fuel economy is almost always obtained at lower engine speeds than that of maximum torque.

LSLGuy

10-28-2005, 06:51 AM

I don't usually try to save gas, but I did try two days ago, just to see what would happen. I drove 150 miles with the cruise at 100kph, used only 3/8 of the tank, and got almost 17 mpg. On the way back, I just drove like normal, had to stop for gas, and got about 13 mpg. At such bad fuel efficiency, 4 mpg is quite a big difference. It's a giant, A4WD, SUV.

Compare that with my normal car. Driving the hell out of it, I usually get 22 mpg when I'm alone. Every once in a while I try to drive like an old man, and I can coax 27 mpg out of the car. Those extra 5 mpg are totally insignificant in such a fuel efficient car (yeah, fuel efficient -- it's a higher-performance, front-drive V8).

So... the key question is, what are you driving, how do you drive, where do you drive, what mileage do you currently get, and what are you shooting for?I suggest your numbers don't quite support your conclusion. In the SUV, you say you can improve your mileage from 13 to 17 with efficient driving. That's a 30% improvement. In the car, the corresponding numbers are 22 mpg, 27mpg, & 22%.

I agree that 30% is more than 22% , but to assert that one is a "big difference" while the other is "totally insignificant" seems a bit off.

Said another way, if you could buy gas 22% cheaper than your neighbors, you'd completely ignore the opportunity, but for a 30% discount you'd do it?

At current prices in my area, 22% is roughly 50 cents/gallon. Speaking just for me, that's not a totally insignificant discount.

Human nature is funny about numbers. Sometimes we unconsciously place very similar numbers into very different mental categories. If we consciously calculate we usually get the logically correct answer. But when we just intuit numbers or comparisons we often goof.

kanicbird

10-28-2005, 07:22 AM

Some things I know of:

Your owners manual may have a section for high speed use tire pressure (like for emergency vechicals), which would be a safe upper limit.

Use full synthetic oil, which you may be able to extend your oil change interval to offset some of the costs.

unless nessessary, NEVER take your foot off the gas, if you need to slow down, do so by reducing pressure, not taking your foot off. You will decelerate slower, but it will work.

Change the sparkplugs and the fuel filter along w/ the air filter.

Look for non-oxygenagated gas.

Use the a/c donwhill, or shut it off going uphill.

Accept a small loss of mph going uphill, make it up going downhill.

Other things like drafting trucks and coasting downhill in N are mainly illegal, but perhaps not everywhere.

Also changing your route to different roads/travel times and combining trips coud help a lot also (well not increase mpg, but less $ spent on fuel).

If you have 2 cars, w/ basically the same mpg use the one that is warmed up (i.e. if you stayed home and your wife just got back, and you need to runout, take the car she just came home in.

metj

10-28-2005, 08:42 AM

unless nessessary, NEVER take your foot off the gas, if you need to slow down, do so by reducing pressure, not taking your foot off. You will decelerate slower, but it will work.

I don't understand this one - why not?

Bosda Di'Chi of Tricor

10-28-2005, 08:46 AM

Check the mileage on the routes you regularly travel. I did, & discovered that by avoiding the Freeway & taking a local road, I could save 5 miles each way, every day. 10 miles of driving a day less. And the side road had a lower speed limit.

I've cut my gasoline bill in half.

I also use my bicycle for local errands.

scr4

10-28-2005, 08:48 AM

What is a Scangauge?
ScanGauge (http://www.thinkgeek.com/gadgets/electronic/72fc/).

Malacandra

10-28-2005, 08:52 AM

Don't take your car out of gear going downhill. It won't save anything worth having and may even lose. Fuel-injection systems, if you have one, can shut off injection any time the wheels are driving the engine, whereas shifting out of gear means you're idling at minimum. And from a safety point of view, you never know when you might need to reapply power.

But this point: Also changing your route to different roads/travel times and combining trips could help a lot also (well not increase mpg, but less $ spent on fuel). gets a big "Hear, hear!". In the computing trade we say "Don't improve the code; find a better algorithm". Use the car less, arrange to spend less time stuck in traffic, car-share, and you'll bring the gas bills down way quicker than adjusting the tyre pressures.

CookingWithGas

10-28-2005, 09:32 AM

I'm sorry, but this is not true as written. Since maximum torque is measured at WOT. . .Thanks for the insights. But wot the heck is WOT?

CookingWithGas

10-28-2005, 09:38 AM

unless nessessary, NEVER take your foot off the gas, if you need to slow down, do so by reducing pressure, not taking your foot off.
Why? :confused:

Accept a small loss of mph going uphill, make it up going downhill.
This may indeed be good for mileage but it wreaks havoc on traffic patterns in heavier traffic, particularly on the highway. Traffic that races down a hill then slows to climb back up causes a wave of braking behind by people who intended to maintain a constant speed, then you get into this pattern that's hard to break up.

crazyjoe

10-28-2005, 09:47 AM

WOT = Wide Open Throttle (basically, "put the pedal to the metal")

Balthisar

10-28-2005, 10:41 AM

I suggest your numbers don't quite support your conclusion. In the SUV, you say you can improve your mileage from 13 to 17 with efficient driving. That's a 30% improvement. In the car, the corresponding numbers are 22 mpg, 27mpg, & 22%.
…
Human nature is funny about numbers. Sometimes we unconsciously place very similar numbers into very different mental categories. If we consciously calculate we usually get the logically correct answer. But when we just intuit numbers or comparisons we often goof.
You're absolutely right of course, and I didn't do the math, and I intuited just precisely as you said. But I think I'll say that one vehicle relative to the other, the difference isn't significant, monetarily. I'll explain (plus, I already inserted foot into mouth -- let me get it back out).

My coworkers that got back from home last weekend say they were paying about $2.50 for gas (wow! it's come down), so I'll use that figure.

I have the option, then, of driving 70 miles round trip per day, of which this is virtually all highway miles. I'll assume for my purposes that it is, and I constantly get 13 or 17 in the SUV depending on driving style (weeks 1 and 2), and 22 or 27 in the car (weeks 3 and 4). In a five day work week, then, for each of four weeks, I expect to spend (yeah, I know, 31% difference in the SUV, 23% in the car):

Week 1: $67.31 in fuel
Week 2: $51.47 in fuel

Week 3: $39.77 in fuel
Week 4: $32.41 in fuel

That's a weekly savings of $15.84 per week if I drive the SUV slowly, and $7.37 if I drive the car slowly. The percentages are still the same, but the monetary differences are vastly different (this is the basis for saying 4 mpg is a big deal on the SUV, but 5 mpg is not significant on the car). Saving almost $16 per week is a big deal to me -- that's almost $825 per year! But when I look at $7+ per week, well, that's a couple of beers. It's under ten bucks -- psychologically, it's chump change. Still, it's almost $385 per year, and I feel blessed that I can ignore it.

The big payoff, of course, is using the car in 100% of all cases. The worst savings -- ragging on the car vs. driving the SUV slowly -- still saves $11.70 per week, which is still 59% better savings (as a percentage) than the difference between driving the car hard and driving it slowly.

Una Persson

10-28-2005, 12:06 PM

Thanks for the insights. But wot the heck is WOT?
Sorry, as crazyjoe says, I meant "wide open throttle". Let me explain a little more why, so my previous answer does not seem brusque.

The point of maximum torque is an important one, in that's it's a place where the bmep (brake mean effective pressure), the piston speed and friction losses, combustion efficiency, volumetric efficiency, and all other factors converge to get you the most torque possible as the engine is running. But that does not necessarily mean it's the most efficienct point, as many of these factors will change with throttle position, and in general if you're operating with a minimum bsfc (brake specific fuel consumption) in mind, there is a another curve that generally seems to peak at between 1000-1500 rpm for many automobile engines (this curve, like a torque and power curve, is specific to each individual engine). Lower engine speeds keep the piston friction losses down, and (all else being equal, mind you) ensure that there is a longer residence time for the fuel/air mixture, thus not only allowing slightly better combustion, but allowing slightly leaner operation as well (thus reducing the sensible heat losses of the engine, and potentially giving a benefit with respect to NOx production too).

Una Persson

10-28-2005, 12:08 PM

Ugh. Please excuse typos in previous post; it's about 50F in this office and my fingers are frozen solid.

pulykamell

10-28-2005, 12:58 PM

Is it true that ignition uses up a large amount of fuel? If so, at what point would idling use more fuel than ignition?

The absolutely uncited figure I remember being hearing on television is that starting the car takes the equivalent of 30 seconds' worth of gas that would be used idling.

crazyjoe

10-28-2005, 02:04 PM

The absolutely uncited figure I remember being hearing on television is that starting the car takes the equivalent of 30 seconds' worth of gas that would be used idling.
I think this was the case with carburated engines, but with today's fuel injected cars, I think it's actuall much much lower than that. Probably 10 seconds or so. I'll see if I can dig up a cite for that.

kanicbird

10-28-2005, 05:21 PM

unless nessessary, NEVER take your foot off the gas, if you need to slow down, do so by reducing pressure, not taking your foot off.

Why?

I don't know, but I have personally noticed about a 2-3 increase in MPG by doing this on more then one car. In general terms it is using the princapal of no sudden stops and starts.

Crafter_Man

10-28-2005, 07:06 PM

Buy a motorcycle.

AskNott

10-29-2005, 12:30 AM

Please bear with me. A lot of this is gleaned from a marginally sane columnist named J.L.K. Setright who was once a driver in the Mobil Fuel Economy Runs. The Mobil runs were the definitive fuel economy standards before the phony EPA computerized dyno tests took over. The Mobil runs were not exactly real-world, either, though. Drivers would shut off the engine and coast when possible, and Setright said he was described as a guy who would rather run into something than hit the brakes. You see, once you use fuel to accelerate, braking burns off that same energy. (see Cooking With Gas, above)

So, the more time you spend touching no pedals, the more fuel you will save.

Imagine a kitten strapped to your gas pedal under your foot. If it takes two blocks to reach 30 mph, that is a victory. Screw those honkers behind you; you're saving America's energy independence!

Tuckerfan

10-29-2005, 12:34 AM

The car company with the most wins in the Mobil Fuel Economy Runs was Studebaker.

zut

10-29-2005, 08:09 AM

I think this was the case with carburated engines, but with today's fuel injected cars, I think it's actuall much much lower than that. Probably 10 seconds or so. I'll see if I can dig up a cite for that.I scrounged up a few cites in this thread (http://boards.straightdope.com/sdmb/showthread.php?t=199799). Oddly enough, of the two cites, one said ten seconds and the other said thirty. Anyway, I suspect you're right about the difference between carburated and fuel injected engines, but I'd have to wager that there's enough other differences between large SUVs and smaller cars, and older and newer autos, and different injection schemes, etc, that there would be quite a range of different break-even points.

I think the general observation that the break-even point is in the range of 1/4 of a minute, plus or minus, is probably a reasonably sound rule of thumb.

Rick

10-29-2005, 10:16 AM

Long time Let's see just how high an MPG number we can get guy here.
In no particular order here is what I suggest
Drive slower. Last year when I moved my daughter back from college, I got a flat tire. The space saver spare was rated for 55 mph. So I drove 210 miles at 55. Got 28 MPG, pretty good on a car where I had never previously seen the high side of 22. There is of course a lower limt to where slower = better. It would vary car to car with areodynamics, engine and transmission combo, but in general I would ball park it on a modern car at about 45-50 MPH, this speed allows the trans to get up into its highest gear, and the engine to be turning at an efficent speed.
Look ahead! Look as far ahead as possible if you see the light is red, take your foot off the gas and coast. The more you coast, the less you use your brakes, the better your gas mileage. You will also have a serious advantage over other drivers of car like yours in that you will buy brakes much less often.
When the light does turn green accelerate slowly.
Use your crusie control Let the computer keep your car at the speed you have decided to drive it won't forget and let your speed creep up.
Tire pressures, set to the max. Look at your tire pressure label, there are usually two sets of pressures listed. A lower (comfort) setting and a higher (performance) setting. Use the higher setting. Subject of course to the max pressures listed on the sidewall of your tire.
Rather than shutting off your car in the drive thru how about this novel idea. Park it at the far end of the lot and walk inside! Several advantages here. First it can often take less time. More than once I have seen car lined up around the fast food joint, so I will park, walk inside and it is empty. I am in out and done faster and for less fuel. :D The far end of the lot also usually has spaces open, so I am not idiling burning gas waiting for somebody to pull out, and there is a much lesser chance of a door ding.
Remove your bike rack, roof rack, yak box or what ever from your car. Less stuff in the airflow = better MPG
Empty the trunk. It takes gas to accelerate all that mass to speed

Mr. Slant

10-31-2005, 08:08 AM

=== jasonh300:
What is a Scangauge?
===

The Scangauge is a device that plugs into your car and pulls information from the computer. You can use it as a trip MPG calculator. You can also use it to poll a number of values in your car's computer and pull error codes.
The manufacturer's website at www.scangauge.com does a good job of discussing its functions. I have yet to see a third-party review that really gave it a thorough description.
Aside from calculating trip MPG, my favorite features are the "GALLONS PER HOUR" and "CURRENT MPG" displays, updated every

=== kanicbird:
Your owners manual may have a section for high speed use tire pressure (like for emergency vechicals), which would be a safe upper limit.
Use full synthetic oil, which you may be able to extend your oil change interval to offset some of the costs.
SNIP
Use the a/c donwhill, or shut it off going uphill.
SNIP
Other things like drafting trucks and coasting downhill in N are mainly illegal, but perhaps not everywhere.
===

Re: tire pressure, you'll want to make sure that whetever you do, you don't exceed the sidewall limits of your tire.
Re: full synthetic oil, you can only extend your oil change intervals substantially if your driving is mostly highway. I've played with the economics of this stuff, and there really isn't a cost savings with synthetic crankcase oil until your intervals pass 15K.
Re: turning a/c off downhill, is that good for your compressor?
Re: Drafting off of trucks, we figured out that in another thread you'd need to stay within 4 feet for it to help. Most of us would probably agree that is suicidal.
Coasting downhill is arguably safe, but I have questions as to whether or not it would actually save gas. Your car uses remarkably little gas at idle or going downhill. Remember your car's clutch is one-way in overdrive, and it provides very little engine braking in overdrive going down a hill.

=== Una Persson
I'm sorry, but this is not true as written. Since maximum torque is measured at WOT, you cannot draw a conclusion that the engine will be as efficient at that same engine speed under normal cruising conditions. I've spent more hours than I care to re-live dyno-testing engines to create maps of power versus torque versus brake specific fuel consumption. It is more correct to say that in general terms, under normal motoring conditions where one is trying to minimize fuel consumption, better fuel economy is almost always obtained at lower engine speeds than that of maximum torque.
another post:
The point of maximum torque is an important one, in that's it's a place where the bmep (brake mean effective pressure), the piston speed and friction losses, combustion efficiency, volumetric efficiency, and all other factors converge to get you the most torque possible as the engine is running. But that does not necessarily mean it's the most efficienct point, as many of these factors will change with throttle position, and in general if you're operating with a minimum bsfc (brake specific fuel consumption) in mind, there is a another curve that generally seems to peak at between 1000-1500 rpm for many automobile engines (this curve, like a torque and power curve, is specific to each individual engine). Lower engine speeds keep the piston friction losses
===

Any way we could get a look at one or two of the resultant charts for fuel efficiency versus RPM?
I've been looking high and low for data like that, and so far I've seen ONE chart of that contains that data. And that was a small gas engine in a hybrid.
Would your second post explain why the CVT-equipped Ford 500, which could cruise down the highway at any RPM, settles down to 1500 RPM at highway speeds?

kanicbird

10-31-2005, 03:57 PM

The space saver spare was rated for 55 mph.

It may also have been the lower rolling resistance of that tire that gave a greater MPG.

Re: turning a/c off downhill, is that good for your compressor?

Your compressor cycles by itself. I would not say turn it on and off every 10 feet, but on a long downhill it has time to run and lubercate itself, I really don't see a problem.

Re: Drafting off of trucks, we figured out that in another thread you'd need to stay within 4 feet for it to help

I have a instant read MPG gage and have noticed going from a ave of 30 to about 33 with a close following, perhaps 3-4 car lengths, by going in the 2 range, which I really didn't do all that long for obvious reasons it was reading about 36.

Your other commments are well taken.

Rick

10-31-2005, 04:58 PM

The space saver spare was rated for 55 mph. It may also have been the lower rolling resistance of that tire that gave a greater MPG.
I assume this was in response to my quote.
In a word no. Look gas mileage sells cars, maybe more now then before, but good gas mileage has always sold cars. If two cars are equal, the one with better gas mileage will sell better. If you could jump the gas mileage from 22 to 28 by changing one tire, then why wouldn't the car maker put four of those tires on the car and then have an SUV that got 46MPG? I guarentee that if you could build a SUV the size of an XC 90 Volvo that got 46 MPG, you would not be able to keep any of them on the lot. The car makers know this, if they could build a SUV that size that got that fuel mileage they would, for the simple reason that they would be able to sell every single one they could build.
Also you are flat ignoring the fact that there is one hell of a lot less wind resistance at 55 MPH when compared to 70-75 MPH.

Danalan

10-31-2005, 05:26 PM

You can safely fill your tires to the 'Max P.S.I.' specification listed on the sidewall. This will likely save you fuel, but also increase the stiffness of your suspension -- the car will ride rougher. Handling shouldn't suffer in normal driving. YMMV. (Hah!)

It is generally more efficient to let cruise control manage your speed, however you might want to manually negotiate hills. You can usually afford to slow a little on short uphill climbs, then gain the speed back naturally on the down side or flat. Cruise control is incapable of modulating this as efficiently as you are.

It is actually better to accelerate steadily and quickly to your cruising speed. Not 'jackrabbit' starts, but using the power band of your engine/ drive-train to get up to speed quickly. You will spend more overall time at efficient speeds/ throttle settings. This works especially well with a manual transmission. Get up to 55 in seconds while rowing through 1st, 2nd & 3rd, and you can jump right into 5th and set the cruise control.

IMHO, the best way to increase gas mileage is to increase your following distance. By doing this, you can avoid braking, and the requirement to re-accelerate. The rule of thumb is 3 car lengths: increase that to 5, and you'll use less gas, and be safer. Note that I'm not saying to drive slowly -- you can pace traffic, just do it from well behind. That way, when they slow down suddenly, you can just coast for a while.

AskNott

11-01-2005, 12:35 PM

kanicbird

11-01-2005, 04:49 PM

Rick, although going slower usually is more fuel-effecient, your spare tire example is flawed,

Some donuts have the same outer diameter as the OEM tires, this is usually on cars that need them to prevent drivetrain dammage. This would increase MPG, again due to less rolling resistance. If they are smaller they basically change the gear ratio and may end up decreasing it.

Rick in a word, nobody buys a car because of the sole reason of mpg. Using 4 donuts would cause harsh handling, fast tire ware, poor ground clearence and a host of other disadvantages, but will increase mpg and also increase acceleration in most cars that can't spin their wheels when floored.

tracer

11-01-2005, 06:24 PM

I drove 150 miles with the cruise at 100kph, used only 3/8 of the tank, and got almost 17 mpg.
Wait a minute ... you measure your speed in kilometers per hour, but you measure your fuel efficiency in miles per gallon?!

Balthisar

11-01-2005, 08:15 PM

Wait a minute ... you measure your speed in kilometers per hour, but you measure your fuel efficiency in miles per gallon?!
Well, I use miles and mpg for virtually everything -- but the speed limit was set in kph, so I used that for actually setting the speed control. I don't wear a fedora yet, so there's no way that I'll drive below the speed limit -- even if it's only 2.something mph!

Regarding low rolling resistance tires, there are cars that come with them. I remember when I was in the market for a Honda Civic. Of course I wanted to EX loaded. But there was also an economiser CX with a different valve timing and low rolling resistance tires the contributed to its vastly higher MPG -- not that I understood the point of trying to save gas in a car that's already dinky and uses next to nothing in gas!

You don't see this too much in full size and larger vehicles, because they cost more, meaning that people that buy them have more money, meaning that the cost of gasoline impacts their financial decisions much less - who cares about 5mpg? Kind of what I mentioned above -- there's a point where the return is so miniscule given your income, you kind of just stop worrying about it unless there's a sea change in efficiency. So, yeah, I found a raison de ser for the Honda Civic CX -- for people in that price range, every mpg helps.

scr4

11-01-2005, 09:05 PM

I don't see why you guys are so critical of Rick's result. Overcoming air resistance at 75 mph requires 150% more power (i.e. 2.5 times more) than at 55 mph. OK, you also get there 36% faster, but even then you've used up 85% more energy for the whole trip.

Rick

11-01-2005, 11:36 PM

Rick, although going slower usually is more fuel-effecient, your spare tire example is flawed, if the spare was on a driven wheel. The space saver spare is smaller in diameter than your original tire, so your speedometer and odometer were off. You were going slower than 55 mph, and you drove less distance than the odometer said you drove.
BZZT not.
The spare was on a rear wheel. The speedo calculation on a Volvo is done by ABS control module. The ABS averages the speed of the two front tires, and then broadcasts this info on the computer net. So no, my speedo was not off.
Secondly, the OD of the space saver spare is within millimeters of what the factory tire is, the space saving is in the width, and weight.

Some donuts have the same outer diameter as the OEM tires, this is usually on cars that need them to prevent drivetrain dammage. This would increase MPG, again due to less rolling resistance. If they are smaller they basically change the gear ratio and may end up decreasing it.

Rick in a word, nobody buys a car because of the sole reason of mpg. Using 4 donuts would cause harsh handling, fast tire ware, poor ground clearence and a host of other disadvantages, but will increase mpg and also increase acceleration in most cars that can't spin their wheels when floored.
I didn't say that gas mileage was the sole reason people bought cars, go back and read what I wrote. Here I will save the trip upthread If two cars are equal, the one with better gas mileage will sell better. But hey, I will go with gas mileage as being the single most important item on some car buyer's list. Want proof? Try this
If you believe that gas mileage does not sell cars that explain why Toyota Dealers have waiting lists for Prius and Ford Expeditions are sitting on the lot?
Hint, it's not because the Prius is bigger, or faster, or carries more. Gee what does a Prius do better than an Expedition? Could it be gas mileage?
As far as my contention that slowing down saves gas, scr4 got it right, there is one hell of a lot less air to push out of the way at 55 then at 75.
And since we are in GQ
cite (http://en.wikipedia.org/wiki/Gas_mileage) Avoid driving at high speeds. Gasoline powered cars operate with maximum efficiency in the highest gear at the lowest speed in that highest gear without engine lugging. This effect is largely due to aerodynamic drag. In highway driving over 80-90 km/h (50-55 mph) (depending on the car's aerodynamics; low for small cars and high for SUV, trucks), the aerodynamic drag will rise sharply and so rises the fuel consumption.
another cite (http://auto.howstuffworks.com/question477.htm) But the bottom line is, if you double your speed, this equation says that you will increase the power required by much more than double. A hypothetical medium sized SUV that requires 20 horsepower at 50 mph might require 100 horsepower at 100 mph.
and a final cite (http://www.fueleconomy.gov/feg/driveHabits.shtml) While each vehicle reaches its optimal fuel economy at a different speed (or range of speeds), gas mileage usually decreases rapidly at speeds above 60 mph.

As a rule of thumb, you can assume that each 5 mph you drive over 60 mph is like paying an additional $0.21 per gallon for gas.
If you are going to continue to insist that my observed real world gas mileage improvement is due to my having a spare tire mounted and was not a result of a lower speed, all I can say is when come back bring cite.

Declan

11-02-2005, 02:58 AM

I don't understand this one - why not?

Just a wag , but I think it has to do with conservation of energy. Starting from a standing stop , it might take an ounce of gasoline to get the car up to highway speed ,but teaspoons to keep moving at highway speed. Reduce the pressure on the gas pedal and you start to slow down, then re-apply a greater amount of pressure to regain your previous speed and the car only uses say a ticture of gasoline.

Using the brake would cause the car to slow to a point where your back to using an ounce of gasoline to regain your speed, do this enough times and your wasting gasoline.

Declan

kanicbird

11-02-2005, 05:33 AM

Rick Your cites don't prove that anyone would buy a new car w/ 4 donuts.

You said it yourself, if all other things are equal someone would tend to buy car w/ better mpg, but using 4 donuts on one car and 4 standard tires makes things unequal, so your statement invalidates itself.

By your own words you have not performed a valid test, the only cite I need is this thread.
http://boards.straightdope.com/sdmb/showthread.php?t=341840

Also there are low rolling resistant tires which are better then donuts but don't seem to really sell on new cars as OEM.

Rick

11-03-2005, 12:56 AM

I never said that anyone would buy a car with 4 donuts, what I said was [making fun of the stupid statement that a 6mpg improvement in gas mileage was totally due to one count it one donut spare tire]If you could jump the gas mileage from 22 to 28 by changing one tire, then why wouldn't the car maker put four of those tires on the car and then have an SUV that got 46MPG? I guarentee that if you could build a SUV the size of an XC 90 Volvo that got 46 MPG, you would not be able to keep any of them on the lot.
If you believe that you could get Prius type gas mileage out of an XC 90 by simply changing the tires, and yet the car makers don't for whatever reason, then your tin foil must be slipping. Dude get a grip. There are not four tires on this planet anywhere that will allow you to get 46 mpg in an XC90. If there was then at least some (read every single one) carmakers would be lining up to put them as OE on their cars and trucks. This is for the simple reason that that great a change in gas mileage would be an unbeatable sales advantage. Anybody that did not put these wonder tires on, would have a hell of a time getting anybody to buy their sleds.
Oh and cute trick of citing this thread :wally
FTR let's look at some of the comments in this thread.
I know that going 55mph will use less gas then going 75mph
Plus comments by Tuckerfan, Chaparral v8, Mr. Slant, and scr4 that slower = better gas mileage. Then on the it's all due to rolling resistance we have you. Your "cite" isn't looking too strong here.
Now I agree that maybe, just maybe my spare tire has a lower rolling resistance. So in that 6 mpg increase maybe 0.1% is due to a different tire, maybe it was 0.2%, but no way it hell 100% of the observed gas mileage increase is due to lower rolling resistance. Hell let's go crazy and say that the rolling resistance contributed 10 times more to the improved gas mileage than I think it did. That means that the improvement due to the one tire was 1% of the measured improvement in fuel mileage (6mpg). for a whopping increase due to that one tire of 0.06mpg. Whoop-de-fucking-do.