Reason I'm asking is that my old set of 18s have treadlife left for one more autoX event. But my 17s are bald now except for the fronts which has roughly one to two more autoX event. I want to see if this will put me in understeer city or has anyone here effectively ran these on a course.

Sizes are 255/40/17 and 275/40/18.

 

will cause electronic problems. I used to run a 275/40-17 front with a 275/40-18 rear. Great combo.

 

BTW... Most folks who race high horsepower cars run a 18 front and 17 rear. The idea is that the taller rear sidewall helps put power down (a little more tire compliance) while the shorter sidewall in front helps with turn in. This is true in the Viper, Corvette, Porsche world.