mileage with new tires

joe_mnJuly 14, 2010

car has trip computer. have had car for 10 years. car gets 26-27mpg on the highway. got smaller tires. 4% smaller. now car gets 30mpg on highway. had them for about 2 months. have hit 31mpg at times. never did that before. so mileage is up about 15% but tires are 4% smaller. i know odometer is off. i can live with that. but why is my mileage so much better?

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john_g

Mileage differences can be accounted for by;
The odometer not measuring correctly.
Lower vehicle profile, as in less wind resistance.
Thinner tires also provide lower wind resistance.
Lower rolling resistance possibly, this can sometimes work against you with the purchase of new tires.

There are many other factors to consider, driving habits as in where you are currently driving if there has been a change. Traffic patterns, climate, all play a role.

    Bookmark   July 15, 2010 at 10:03AM
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joe_mn

original tires were 225/60-16. current tires are 245/50-16. used also. were on another car we had. i zero computer when i get on road. drive 20 miles, interstate. flat roads. 26mpg old tires but 30mpg new tires, same weather, same speed, same road. maybe old tires had more stick? low priced a/s stuff. current tires are hp a/s. minimal tread left. will be off car this fall.

    Bookmark   July 15, 2010 at 10:38AM
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jemdandy

The original tire (225/60 - 16 inch) has a nominal circumference of 83.66 inches. The nominal circumference of the replacement tire (245/50 - 16 in) is 80.56 inches. The replacement tire will turn 3.8% more revolutions per mile. Tread wear on either tire will modify these figures.

    Bookmark   July 16, 2010 at 12:47AM
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joe_mn

stock size tires perform well in snow. actually have fairly good bite. not so good in cornering though. seem to slide on dry pavement. the summer tires have very stiff sidewalls. maybe increased mileage is due to less rolling resistance? very shallow tread. down to 1/4" tread.

    Bookmark   July 16, 2010 at 11:15AM
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reekola

Does odometer "read" mileage based on certain tire size revolutions per mile? If so, wouldn't a smaller tire do more revolutions per mile, as jemdandy says, resulting in the odometer thinking more mileage is being driven than actually is? Is so, then maybe the mpg thinks more miles per gallon are being driven when in reality it's not. Just a thought - not sure how odometer and mpg calculator works.

    Bookmark   July 22, 2010 at 2:51PM
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joe_mn

computer gets info on distance traveled by counting tire revs. tires are smaller than stock size so speedo reads 4% wrong. but also calculates mpg by distance traveled and fuel used. should tires with identical rolling characteristics vary mpg only by the different diameter? yes, car is 1" closer to ground. maybe better aerodynamics? but mileage is 15% better, not 4%.

    Bookmark   July 24, 2010 at 11:13AM
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