Tire diameter and gas mileage

What effect would a larger diameter tire have on gas mileage? It seems to me that the larger diameter would roll farther for each revolution. So that it would result in better mileage. But then there’s the increased mass (provided it was the same design made bigger and not some lighter design). It probably takes more gas to get that greater mass moving in the begining. But is the greater mileage from the increased diameter offset by the greater acceleration power need to defeat the greater mass?

Having a larger tire diameter is essentially the same as having a higher gear. Mileage improves, acceleration worsens.

As with everything in life, there is a point of diminishing returns. To see a noticeable RPM drop you’d have to add another 3" or so of diameter. Even if you could fit such a tire in your wheel well, this could drop your engine’s operating RPM too low which would result in engine damage due to “lugging” the engine. Every engine is designed to operaate efficiently in a certain RPM range. Operating outside that range (high or low) is not good.

Going up a size or two with your rear tires won’t result in enough of a change to effect MPG, although your RPM’s will drop a bit.

Another factor…if you don’t change all four tires you’ve created an issue with your spare. If you’ve got a car with a limited slip/positraction rear end, and you use a “short” spare, you’re looking for trouble.

While tire mass will have a small affect on mileage, the biggest contribution will come from engine RPM (rotations per minute). Your “Rolling more per revolution” argument is incorrect. When the tire size is larger, the engine needs to put in more torque to move through the same angular displacement - essentially meaning that your mileage should stay constant. However:

Internal Combustion engines have a very narrow band of RPM where they are efficient. In fact outside the band IC engines produce very little power output and are pretty useless, which is why we need gears to keep the operating RPM within the usable range for a wide variety of road speeds.

Now cars are designed keeping this RPM range, gearing ratios and tire sizes in sync to provide optimal mileage around the speeds where cars are mostly driven, such as just below speed limits, which are fairly uniform. By changing tire sizes, you’re throwing these design parameters out of whack.

Changing your tire diameter, whether you’re making it larger or smaller, without changing your driving habits, will result in a reduction in mileage.

Since the engines are tested for tolerance and longevity around the designed parameters, you might even seriously damage your engine if you change your tire diameter far enough.

(Bolding Mine)

I’m going to quibble a little with this, as it really depends on what side of your engine’s brake-specific-fuel-consumption “bullseye” you’re driving at. See example here. While it’s generally true that under highway-speed conditions when you’re using the gas pedal gingerly, one can get efficiency improvements by lowering the gear ratio (essentially going into a higher gear, dropping RPMs, and moving leftward on the BSFC map), but you’d end uplosing efficiency while you’re driving with a more open throttle (i.e, accelerating) at a lower speed. The engineers have the gear ratios designed for a reason.

On an unrelated note, you can improve gas efficiency by inflating your tires to a higher pressure. At a higher pressure, the tire deforms less under the weight of the car, which results in less rolling friction. Basically, there’s a small flat spot on the bottom of your tire where it contacts the road, and moving that flat spot around the tire as it rolls uses energy.

The tradeoff, of course, is that by overinflating your tires, you’re decreasing their longevity, and might even cause them to fail catastrophically.

A particular driving condition requires a constant power, not a constant torque. That means lowering the gear ratio drops engine speed and increases load at the same time. While your general point that lowering the gear ratio doesn’t increase efficiency everywhere on the map holds, your linked diagram shows why efficiency in general improves: there’s only a thin band of low speed, high-torque points where a lower ratio actually hurts. Furthermore, I suggest that that area of the map is likely used relatively rarely, as a driver with that kind of lead foot is more apt to downshift.

Moreover, I suspect it would be hard to construct a driving cycle where the efficiency losses at those points predominate gains at other points, so I think Gary T’s point holds in general.


Depends what you mean by “over” inflating them. My car specs around 32 psi for the tires, but the sidewall rating on my tires is 51 psi; I typically keep them inflated to around 45 psi. Haven’t particularly noticed a loss of traction or unusual wear, and since I’m well under the sidewall rating, I’m not worried at all about catastrphic failure.

If you change tire size without doing anything else, your speedometer will be off, as will your odometer if you’re trying to use it to calculate your mileage.

A last point is that if you increase tire size and don’t compensate the odomoter, it will read low. So even if you’re actually gaining gas mileage, you may not be able to see it in your calculations of [distance traveled as shown on the odomoter] / [gas cunsumed].

In a panic stop you would likely notice the difference, and on wet or otherwise slippery surfaces. Your car suspension is expecting a certain tire inflation to work optimally. Some tweeking is well within the safety margins, a lot isn’t. But knowing where that line is requires more knowledge than I possess.

I wanted to add a little further to this.

It might depend what your goals are. Do you drive at high or low speeds? Do you drive at steady speeds, or do you frequently slow down and accelerate aggressively?

The U.S. government’s EPA fuel economy procedure never gets over 60mph, even on the highway cycle. I can’t prove it, but I think it’s reasonable to believe that auto manufacturers might engineer their cars to perform well in the EPA tests, both to meet the government’s fuel-economy requirements, as well as boost sales.


If you conduct all your driving on the highway at 80mph, or conversely, you stay in town with many stops and never attain 40mph, it might be beneficial to adjust your gear ratio (or tire size). However, there would be a lot of costly trial and error to see if there’s an improvement, or if maybe things get worse.

Here is one of the many RPM calculators you can find on-line:


You can get your tire specs (diameter) at Tire Rack: www.tirerack.com

Caution w/regard to published tire diameters. The manufacturers’ diameter numbers, which should be identical regardless of brand, will not be the diameter of the tire mounted on a wheel and mounted on the car. When using a calculator that calls for tire diameter it’s best to measure the radius from the center of the hub to the pavement and then double the number.

Correct me if I am wrong but I was always under the impression that gear ratios for the rear end were calculated as follows: 3.73:1 was 3.73 turns of the drive shaft for every one turn of the tire. If this is correct, and you increase the tire size, you increase the circumference and therefore increase the number of turns of the drive shaft to one turn of the tire. Now you have a gear ratio of 4.11:1, as an example, which is a lower gear ratio which is better for low end than it is for high speed.

Changing the tire diameter won’t have any effect on the ratio of the turns. That’s determined entirely by the gears connecting the shafts together.

If lowering the gearing was really an efficiency gain whose deficiencies would not be experienced by most drivers, why wouldn’t manufactures simply design the car with lower gearing in the first place?

Far and away … driving style is the biggest impact on gas mileage on any particular rig … drive a little slower … accelerate nice and easy … think “lil’ ol’ lady” …

My citation is from Consumer Reports “Tips on How to Save Money on Gas” … and they go on to say tire inflation has very little to do with gas mileage … I’ve read other places that this does have a bit of an effect …

We’re long overdue for Unca Cece to rent a Corvette and spend a few weeks gathering scientific data in this matter …

You’re wrong. :slight_smile:

You have to change the rear end gearing to make that gear ratio different. Increasing the tire circumference cannot change if from 3.73, that stays constant. What changes is how far the vehicle travels with one revolution of the tire (and therefore with 3.73 revolutions of the driveshaft) – it will cover more ground. This could have the same effect as changing the rear end gearing to, say, 3.59 (number made up to illustrate principle) – it takes less turns of the driveshaft (and thus the engine) to travel a given distance.

They have a few (dozen) other criteria to consider, and have chosen the compromises that they believe are optimal for that whole big set of variables.