Questions about how to get better mileage.

What are some ways for me to get better mileage out of my car? I know to keep the windows up and AC off. 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?) and to make sure that you have a clean air-filter.

What about driving speed? I know that going 55mph will use less gas then going 75mph, would going slower yield even better results, is 25mph more efficient than 35 or 40?
Is it true that you burn up a lot of gas when accelerating?
Are there any additives that would give me better mileage?

Any other tips?

I forgot one.

Does using cruise control more efficiently conserve gas then manual?

I read in a trivia book once that 35 - 40 MPH is the most fuel efficient speed. Not sure if that is right. Keep you speed as constant as safe driving allows. Don’t do rabbit starts at red lights and stop signs.

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

Lots of things can affect your gas mileage. If you want to improve your mileage, then take everything unnecessary out of your car (more crap to haul around, means more gas gets burned), strip things like luggage racks, bike racks, etc., off the car. Get a tune up (if it’s been 100K miles since your last one [assuming you drive a car of recent make]), if your car doesn’t have a front spoiler/air dam, add one of those.

Driving over 45 MPH means that your car’s aerodynamics can start to negatively impact your mileage.

Avoid jack rabbit starts, and if you’re going to be sitting for longer than a minute or two, turn the engine off.

You do all that, and you’ll save a couple of gallons a tankful.

Starting a car doesn’t ‘waste fuel’, so turn the engine off while waiting at drive thrus for fast food, the bank, and anytime you expect to be somewhere for a while.

It is impractical and unwise to turn off the engine in normal driving on normal roads and might be illegal. It would save gas if the car engine was shut off, rather than idling. It is impractical in far too many instances.

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.

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.

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 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"

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:

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.

What is a Scangauge?

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.

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.

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.

I don’t understand this one - why not?

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.


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:

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.

Thanks for the insights. But wot the heck is WOT?

Why? :confused:

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.