Anyone want to venture a guess as to why flashing my 2000 Camaro computer with the 2002 Camaro OS causes the tach and speedo to quit working?

It makes no sense. The speedo comes from the power train module (PCM?) and the speedo comes from the class 2 data link. So do the coolant temp, fuel level, volt meter, and oil pressure which DO work.

The only change to the car since is shelled the 5.13's is a new 4.88 ring and pinion, but that surely can't cause the problem.