I am close, but missing something with the speed calculation. I have a 40 tooth VSS on the back of the transmission (before the transfer case for a 4WD). I have 34x12.5x15 tires with a 4.10 gearing. By my calcs the following information should be correct.

Gear Tire Circ Tire rpm Trans shaft rpm VSS Teeth VSS PperMile
4.10 106.12 597.06 2,447.95 40.00 97,917.83

However, when I enter in these values, the reported speeds by the PCM are not close. I need something like 175000 PPM to get close to the actual speed. Can anyone help with what I am missing? Also, I thought I read somewhere that the trans shaft rpm value is what drives the trans shift behavior. Is that true?

Thanks for any pointers!