Originally Posted by Speeddemon420
Thanks for the advice. Would I be able to run a 255/40/18 with the stock suspension and the rear fenders rolled?
Definitely don't want to run 255/40's, that's an extremely tall tire over 26" diameter. Stock WRX tire is 25.3" diameter, while stock STi is 25.7" diameter. Due to the WRX's taller gearing, going with a slightly shorter tire like the 255/35 (25") will help gearing. And the STi in turn will benefit from a slightly taller tire as it has fairly short gearing. So if you have an STi, the size you mentioned would work fine. If you own a WRX, you want the 255/35s.
If you do go with the taller tire though, you will def need to get more aggressive with the rolling, and potentially even have to roll the fronts though I'm not sure on that one.