I do 35 all around for normal driving (slightly higher tire pressure improves gas milage) and 40 front 32 rear for autocross. Your tire's maximum pressure rating will be printed on the side of the tire. Mine are 44psi so I feel comfortable doing 40 psi for a limited time. For normal driving, do not come within 5psi of the maximum rating because as the tire heats up on the highway, the air inside will expand, raising the pressure... so if you put in 35psi at the gas station, if you go for a drive, your tires may actually be at 42psi depending on conditions and what kind of driving you're doing. If your tire's maximum rating is 40psi, you're at risk of damaging the tire and possibly causing an accident.