IMO, the correct answer is a combination of the two numbers-for instance, on the Cherokee in my sig I have slightly oversize tires, 235/75R15 Geolandars, that are rated 50 PSI max. The tire placard for the OEM 225s suggest 33 PSI on both axles-but the fronts wear heavily on the shoulders (as do most solid axle 4X4s) & squeal on turns at that pressure. So I have 40 in the front & 35 in the rears, works fine-& if I pull the utility trailer the rears are 50.