The proper way to calibrate the speedometer is to take the rollout distance of a mounted tire/wheel combination. To do this mark your tire at the point it hits the ground and the ground. Drive straight forward until the tire makes two revolutions and the mark on the tire is now hitting the ground again. Mark the ground and then measure between the two marks on the ground. Take this measurement and use the following formula to get the dia. of your tire/wheel combination.
An example would be as follows
(208"/2)/3.14159265359 = 33.10423"
If your programming tool is working properly this will get your speedo perfect!
Mainemax-If you have a programmer like a quad stealth, predator, hypertech, or bullydog they can all set tire size. They can't alter shift points for the allison but they can set the speedometer/odometer and abs for larger tires. And of course add more power.
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