Just wondering why GM decided to design their computers to shift using speed rather than RPM? It would seem to make more sence to do it that way instead because if you put different tires or different gears, it throws everything off.

 

I'm guessing so that the tranny won't shift during loss of traction... (rain, snow, burnout) just a wild guess though....

 

The PCM (unless tuned otherwise) shifts only when BOTH road speed AND RPM is reached. One of the main reasons was explained by sabre81, other reasons being loss of one of the signals (either RPM or speed output) and then the computer still has a backup for shifting from the second variable.

 

I know this thread is old, but is there a way you can tune the computer so that the shift points are at a low speed, and the RPM shift point is higher than the speed shift point?

I don't know if that makes sence, but could you say program the computer to upshift from 1-2 at like 20 MPH (which may only be 4000 RPM) and have the shift RPM at 5800 like it should be? That way if you use taller tires, you won't be holding the gear for so long and you will be shifting at the correct RPM?

 

easily, just edit the shift point tables, if its WOT your talking about, change the kickdown/WOT shift tables for RPM and speed to where you want, if its from one of the other tables (below WOT setting) just put in the speed based on throttle for each gear shift.