18 would be ok as long as the tyres and the wheels diameter is the same as the 17inches, which mean the 18inches would need thinner tyres. If you put it in simpler terms, the smaller the wheels the quicker acceleration although lesser top speed, and too big will give you slow acceleration and a higher top speed. You just have to see for what situation your car will be in. I heard 17 inches are the best, but dont know for sure...