[grampa_ss]

Originally Posted by cerino2000

Exactly why a smaller tire would make the car shift at a lesser RPM. In stock form let’s say the 2-3 shift happens at 60mph and in stock form let’s say that is exactly at 6500 rpm. In stock form that’s 726 rpm at the wheel assuming stock tire diameter 27.77. So if the cars computer is in stock form, it says when that tire is spinning at 726 rpm, we must be at 60 mph...time to do the 2-3 shift. If we go with a 1” smaller diameter of 26.77 and the car is still in stock form, it’s looking for that 726 RPM of the tire to know when to do that 2-3 shift which in stock form we said equated to 6500 engine RPM. The problem is that with a 1 inch smaller tire, 726 wheel rpm is only really 57.8 mph and the engine RPM that requires 726 revolutions at the wheel in 2nd gear with the smaller tire will be less than 6500 rpm thus short shifting happens.

At least that’s my theory!

Close, but not quite. It's the exact opposite. These A8/A10 shift by MPH, not RPM. If the car is programmed to shift at lets say 60 mph (6500rpm) with the stock tire. If you use a shorter tire, it'll take maybe 6800 rpms to reach 60 mph and you'll bounce off the limiter by the time the shift actually happens.

I know this because I went to the track with shifting set at stock tire height, but at the track I switched to my 26" DR. I hit the limiter on my 1-2, 2-3, 3-4 shifts. Informed my tuner and 1st thing he said was "are you using a shorter tire?". He sent me a new tune with lower mph shift points.