This one calculates tires sizing as well as wheels offsets all on the same page. It even gives end result gearing calulations based on the wheel/tire combo. Anyone know what the auto and manual gear ratios are for the Scions?

http://gs.tolan-hoechst.com/tirecalc.htm

 

try this one....with graphics

http://www.miata.net/garage/tirecalc.html