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Tire diameter vs. speedometer question

This is a discussion on Tire diameter vs. speedometer question within the Computer & Tuning forums, part of the LSx Technical Help Section category; In HP Tuner if you increase your tire diameter does your speedometer slow or go faster? I used to know ...

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    Tire diameter vs. speedometer question

    In HP Tuner if you increase your tire diameter does your speedometer slow or go faster? I used to know but if I remember right the logic is opposite from how it actually changes.
    Last edited by Larry; 01-08-2010 at 01:25 PM.

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    NY Representative basballny2's Avatar
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    not sure but thinking about it logically, if you get bigger rims the car is going to read its going slower than it actually is.......but thats just my opinion, subscribing to learn something as well

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    Member Benner's Avatar
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    Larger diameter means you'll actually be going faster than what your speedometer show. I.E. (when your speedo says 60 you're actually going 65)

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    Agree. Each full rotation of a larger tire travels a greater distance than a smaller tire.

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    Quote Originally Posted by Benner View Post
    Larger diameter means you'll actually be going faster than what your speedometer show. I.E. (when your speedo says 60 you're actually going 65)
    True in real life but not sure that is the way HP Tuner recognizes tire diameter in their program or should I say GM's PCM.

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    NY Representative basballny2's Avatar
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    Quote Originally Posted by Benner View Post
    Larger diameter means you'll actually be going faster than what your speedometer show. I.E. (when your speedo says 60 you're actually going 65)
    i was right!

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    "The Snake Skinner" Smokaholik's Avatar
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    so how do i correct my speedo for the larger diameter?

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    Quote Originally Posted by Smokaholik View Post
    so how do i correct my speedo for the larger diameter?
    You reprogram the PCM -- can be perfromed with a handheld or any of the commercially available tuning programs. You can also obtain a mail order tune from Frost (site sponsor) in which this can be accomplished along with his other performance enhancing tweaks.

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    Quote Originally Posted by pajeff02 View Post
    You reprogram the PCM -- can be perfromed with a handheld or any of the commercially available tuning programs. You can also obtain a mail order tune from Frost (site sponsor) in which this can be accomplished along with his other performance enhancing tweaks.

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