tires for my new 15x4 rims

25" is the standard so 26 and 27 inch high tires will effect the rear end gear and raise the ratio which is lowering the number. What gears will you be running and which transmission, these will help you decide what exactly your speed will be at what RPM.

I know I don't explain everything perfectly but I do know gearing. I run 3:55 with 245/60/14 rear tires ( 25.57" ) so @ 3000 rpm, I am traveling 63 mph..