I took this from Allison's website:
Q. Speedometer / Tires - What effects will big tires have on transmission operation? A. The TCM calibration is based on factory tire size and rear axle ratio. Changing either of these parameters will have some adverse impact on transmission operation, including features like grade braking and shift stabilization. For instance, we have noted that when P285 tires are installed that the transmission generally does not shift to 5th range when towing heavy loads.
My question, can my dealer change the TCM calibration and the speedo calibration to compensate for my new 285's or is the TCM calibration a fixed thing / and is it necessary?
Q. Speedometer / Tires - What effects will big tires have on transmission operation? A. The TCM calibration is based on factory tire size and rear axle ratio. Changing either of these parameters will have some adverse impact on transmission operation, including features like grade braking and shift stabilization. For instance, we have noted that when P285 tires are installed that the transmission generally does not shift to 5th range when towing heavy loads.
My question, can my dealer change the TCM calibration and the speedo calibration to compensate for my new 285's or is the TCM calibration a fixed thing / and is it necessary?