SHELBY GT 350 Member
Joined: Jan 2011
Location: Central Massachusetts
An easy place to start, is look at the tires that American Muscle sells with those wheels as a wheel/tire package.
Generally, you want to match the overall diameter of your original tires, so the speedometer will be correct / won't need to be corrected. With 20" wheels, that means you will have a very low sidewall. Usually American Muscle has this all figured out, so that's why I say they are a good place to start. Also look around for the on-line tire size calculators; they are posted in some of the threads on here but I don't know exactly where off the top of my head.
| 2010 GT coupe | Daily Driver with some Steeda and GT500 take-off stuff for the occasional track day