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How much do different tire sizes affect mpg?

Discussion in 'Gen 2 Prius Main Forum' started by StephenJ, Jun 5, 2018.

  1. StephenJ

    StephenJ Member

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    Hey guys! So I’m not much of a math person but have a question. As I was tire shopping I noticed that the OEM tires that came on my 2009 Touring (195/55/16) have a spec of 854 revolutions per mile. Then I noticed that the current tires on the car which are the same size have a spec of 844 revolutions per mile. I am looking at a new set of tires in 205/55/16 and rated as 834 revolutions per mile.

    The stock size is 24.4 diameter (which if that’s a light number I don’t see how they can have different rev per mile 854 vs 844). The larger tire is 24.9 which is the 834.

    Will the larger tire make my mpg reading show high or low compared to the OEM tire size?
     
    #1 StephenJ, Jun 5, 2018
  2. bisco

    bisco cookie crumbler

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    a larger diameter means the car has to travel farther to report 1 revolution to the computer. which means over the same distance travelled, the computation will result in lower mpg's. i think.
    however, this has nothing to do with actual mpg's, which could be affected by a different tire and size.
     
    #2 bisco, Jun 5, 2018
  3. StephenJ

    StephenJ Member

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    Yes, I think it may be okay on actual mpg though because the tire tread is the same width and the larger tire is actually 2lbs lighter than the ones I’m replacing. So I think it’ll work out okay.

    I’m mainly just trying to gauge how this will affect the mpg reading that the car calculates.

    I’m thinking with each axle rotation I will actually be going farther now than stock so when the car shows a mile I really went say 1.05 mile or something. This means if I fill up and go through a tank of 10gallons of gas the car will think I went less miles on that fuel than I actually did and so read low mpg. Which Bisco, I know that what you said....I’m just thinking outloud to try to get it straight in my mind. Lol

    Am I thinking about that right?

    So if my new tires are 834rpm and the original are 854rpm that is only a 2.5% difference so it’s only going to be off like 1.x mpg low. Does that sound right?
     
    #3 StephenJ, Jun 5, 2018
  4. bisco

    bisco cookie crumbler

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    i think you're right. i think my head hurts.:p
     
    #4 bisco, Jun 5, 2018
    Mendel Leisk likes this.
  5. 09Prius2

    09Prius2 Member

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    A lot of factors.

    Bigger tire means slightly higher ride height, ground clearance, worse aerodynamics and rolling resistance etc...

    Theoretically a larger diameter tire increases final drive ratio, increasing mpg, but no effect with a cvt transmission that always operates at peak rpms for mpg.

    Probably won't make much difference in optimal weather and environment.

    Bad weather you'll get worse mpg. But better traction.
     
    #5 09Prius2, Jun 5, 2018
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