I have read on this forum that the ideal tyre size for rx8 wheels is 225/45/18. I have found some 225/40/18 tyres on ebay and wondered if they would be ok on the rx8 rim.
It will just increase the profile by 11.25mm giving a 70mm difference in circumference.
Shouldn\'t be a problem unless your car is so low that 1.25cm makes all the difference. Speedo will over read still, but by a little more than compared with the 225/40/18\'s.
Mark.