Illiniwek

Right. The 245's on the tire I wanted had a much larger diameter than the original S02's. That is why the speedometer would have been off by about 4%. At least, that is what it says in the sticky topic on rear tire size at the top of this thread.

PJCC

I belive the driver's weight is to blame. Find someone who is about youre same weight and let him (her) ride shotgun with you. If the S is as succeptible as I think to weight and balance, the addition of a passenger should balance the left/right weight distribution and even out the overall pressure exerted over the rear tires.

Other possible factor:
- very small variation on the rear tires traction due to tread imperfections/freshness/chemical composition
- very small variations of surface traction due to road surface imperfections/freshness/chemical composition
- re-calibrate your "butt-dyno".

As long as both tires on the back are the same model and width (225, 235, 245), traction should be equal.