WOT = Wide Open Throttle (basically, “put the pedal to the metal”)
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.
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).
Ugh. Please excuse typos in previous post; it’s about 50F in this office and my fingers are frozen solid.
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.
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.
Buy a motorcycle.
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!
[Hijack]The car company with the most wins in the Mobil Fuel Economy Runs was Studebaker.[/Hijack]
I scrounged up a few cites in this thread. 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.
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. 
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
=== 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?
It may also have been the lower rolling resistance of that tire that gave a greater MPG.
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.
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.
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.
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.
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.
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.
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.