tire width has nothing to do with wheel diameter. A 225 will fit best on something between a 7 and 8.5 in
wide rim, depending on manufacturer. Hankook Ventus Z212s and Fuzion ZRis, for instance, work best in 225 width on 7 or 7.5" rims. A 205 fits a 6-7" rim, in general. Usually you find 16x7, 17x7.5, and 18x8 wheels, but other sizes are made as well. If you have an 18x7 wheel, 205s might be a great choice. However, odds are it's an 18x8, in which case the tire will probably be too narrow. However, it will mount and be safe. The Proxes 4 is a very wide tire foir a 205 and
http://toyo.com/tires/tire_specsheet.cfm?id=18lists an 8" wide rim as just fine. It would be VERY crisp, but probably just wouldn't have enough rubber on the road for your car. 225 is a much better size for 8" wide wheels, and for our heavy car. 225/40R18 would be perfect.