I used to run a 245/40/19 Dunlop sp900 on the stock rears. It gave the car a very aggressive stance and made the rim look wider. I am currently running a 275/30/19 on 19 x 9.5 RUFF racing wheels. Next i plan on running a 235/35/19 for the "stretch and poke" look used in Germany. I say if you want to use a thinner tire, go for it, it will give the x-fire a much more aggressive look.