Dick, Underinflation is totally related to load on the tire. The ST225/75R15 D rated tire can most certainly be run at 50PSI as long as the load on the tire does not exceed 2150 #s (per Goodyear). In fact Goodyear shows cold inflation pressure all the way down to 15PSI (1060# loading) for the ST225/75R15 D rated tire.