I have an aftermarket rims 18x8.5 and I recently bought potenza SO-3 Pole position, here are the sizes..
QTY SIZE & DESCRIPTION AVAILABILITY PRICE EA.
2 255/45YR-18 Bridgestone Potenza S-03 Pole Position In Stock $214.00
2 245/40YR-18 Bridgestone Potenza S-03 Pole Position In Stock $218.00

Based on the tire calculator http://toy4two.home.mindspring.com/tire.html
overall error was 2.1%, you guys think its ok?