The back end of these mustangs are so light, I run 20 psi in the rear and 30 psi in the front for my 68 coupe. I have BFGoodrich Radial T/A's. The best way to tell is how the tire wears. If it wears in the middle, the pressure it too high. If it wears toward the outside, the pressure is too low. Plus, I get better traction running 20 psi in the rear.