[ROUSHAC]

Try 24 psi to start with.

Assuming you have rims that are the correct width for the tires try this. Rub some chalk across the tread and after driving a short distance in a straight line (like the length of your driveway) see if the chalk wore off evenly or left some areas with chalk on them. If there is chalk in the middle, you are too low on pressure. If there is chalk at the edges, you are too high.

Lower pressures will give you a more "comfortable" ride on rough roads, but you don't want to go really low on the psi or the tire will overheat and possibly come off the rim or cause a blowout. For "spirited" driving most folks go a bit higher than the 24 psi.

On the street, I run 24 psi with similar size tires, others run 22, still others run 30. For drag racing I go as low as 18 psi. It's a judgement call on what compromise is best for your driving habits.