Is changing to shorter tires bad for rear differential?

Discussion in 'Brakes, Wheels and Tires' started by ScottyD_GT, Aug 31, 2018.

  1. ScottyD_GT

    ScottyD_GT TMO Intermediate

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    I just upgraded my 275/40/19 rear tires to 305/30/19 which are 1.5" smaller in diameter (actually replaced the front 255/40/19 with the 305's as well to make a square setup). A friend suggested going to a shorter tire like this is bad for the differential. He also suggested the car is setup for staggered so I should keep that to decrease front wheel rolling resistance/increase cooling to brakes. This setup is going to be for road course/track use.

    The PP2 S550 has 305/30/19 square setup, but I don't know if they tune the car differently. I've also read a square setup is better for the track, even though my car came staggered. I feel like gaining front grip at the expense of rolling resistance would be favorable.

    I guess my main question is will the 305/30/19 setup create issues with the differential? What do others think?
     
  2. JAJ

    JAJ TMO Addict

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    Differential will perfectly fine so long as the tires on both sides of the car are the same size. The only effect will be on your speedometer, which will be off by about 5.5% - when it reads 100MPH, you're only actually going 95.
     
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  3. racer47

    racer47 All I got left is fast

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    You need smarter friends :)
     
  4. ScottyD_GT

    ScottyD_GT TMO Intermediate

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    Good, that’s exactly what I thought. I guess this also means flat out acceleration will decrease b/c the same engine RPM will equal lower mph.

    Is this right?

    In general, is it better to give this up in favor of more overall grip with the wider tires?
     
  5. ArizonaBOSS

    ArizonaBOSS Because racecar. Moderator

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    Ignoring the weight increase of any wider wheel and tire setup, you will actually accelerate FASTER with the smaller tire, assuming you don't spin. But you will reach redline in each gear faster as well, so you will do a bit more shifting.
     
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  6. ScottyD_GT

    ScottyD_GT TMO Intermediate

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    Ok good. FYI here's the warning in the '18 manual about using different size tires than OE on the rear axle w/ torsen diff. Doesn't sound like y'all think this will matter, but the warning is there:

    "Extended use of tire sizes other than the manufacturer’s specified tire size on these axles could result in a permanent reduction in effectiveness. This loss does not affect your normal driving and should not be noticeable."
     
  7. Mad Hatter

    Mad Hatter Gotta go Faster

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    The word COULD sounds like ass covering more the anything else.

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