Can larger diameter tires improve highway gas mileage?

[Sutherfam]

I have a 2004 Freestar and do mostly highway driving and am curious if I can improve my Miles per gallon if I put on slightly oversized (larger diameter) tires? Such as a 215-70-16.

Thanks!

[theoldwizard]

Probably not enough to notice.

At the same time your speedometer will be reading slower than you actual speed (great for getting speeding tickets) unless you reprogram the PCM.

[glenroc]

your MPG might actually be worse. your vehichle has been geared for the engine/tranny/tires that came stock. if you put larger tires not only does it affect your speedometer(got a few tickets myself), it also takes more power (gas) to turn the larger tires

[BroncoJoe19]

I had a '67 Cougar with a 289 in it. I put 15 in rims on her, in place of the 14 stock rims for the same reason. I don't think it made a difference, although it may have decreased my low end power. I apoligise that I did not make an actual study of it to be able to report actual results.

[chukarhunt]

larger tires typically reduce your torque at the wheels(just a little), because it takes more effort to get moving. Larger tires could help if you kept your foot out of the gas pedal. Around town not so much, but on the highway I can see them helping in the long run. I put oversized tires on my 81 ford, but I still get 8 mpg on a good day! lol

[tecky11]

i know that feeling !! i drive a 71 international with lt 248/85/16 tires and get 10 regardles of it being full or empty

[khooper]

Remember that wheel weight (this includes tire weight) count two times as normal weight. Also bigger diameter will mean that your car will be sitting higher up in the air and you don't want to destroy your aerodynamics.
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[RoyM]

I had a Dodge p/u with 16" wheels, it originally came with 15" but the previous owner changed them likely wanting to improve mileage. The transmission would shift down at the mere sight of a hill and it was useless towing.
I was amazed one trip, I was in a hurry and cruised the four lane at an indicated 75 (the gears had not been changed). Yeah I was pushing my luck. I got about 10% better mileage than I did at 60 where the engine was turning 1800 rpm and obviously lugging. I thought the gauge was lying and stopped for gas to make sure it was o.k..

[jndejure]

Yes, you will get better mileage, longer tire wear and a smoother ride. If you increase the tire diameter by one inch you will move more than three inches farther for each revolution of the tire. This is much easier than installing a taller final drive ratio in the differential/transmission but you will get a little slower get up and go from a dead stop. Once moving your transmission will make up for most of the loss which will not be large to begin with. And, as for speedo error, most speedometers read a little high to begin with. So, a small increase in tire diameter will bring the speedometer reading closer to or slightly more than the actual miles per hour. I think you can handle the fact that your car is now going a couple of miles per hour faster than the speedometer is actually reading at highway speeds.

Tire pressure is also a big factor in mileage. The average front wheel drive passenger car should be running around 38psi front - 32psi rear (the axle with the most weight should have more air pressure). You can thank the legal system for instructing even tire pressure front and rear. There's always somebody who will forget that the axle with the engine and transmission over it should have more tire pressure. Remember the Corviar with a rear engine in a small car? GM correctly cited tire pressure to be 16psi front, 26psi rear considering that the flat six cylinder motor extended in back of the rear wheels.