Tire diameter vs. speedometer question

[Larry]

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.

[basballny2]

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

[Benner]

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)

[pajeff02]

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

[Larry]

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.

[basballny2]

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!

[Smokaholik]

so how do i correct my speedo for the larger diameter?

[pajeff02]

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.

[35th-ANV-SS]

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.