I'm running 225 40 18s on my RX8 wheels. You could also go with 235 40 18s which are the stock RX8 size. Anything smaller than that would be too small. Even the 225s are a bit narrow for the RX8 wheels, and the Michelin Pilot Sports that I'm running now are a bit stretched. I had Continental ContiSport Contact 2 tires before, and even though they were the same size, they seemed to fit a bit better. The ride with the Michelins is better, though.
