Go off the tire sidewall. Sounds like it left the factory with either bias ply or load range B tires and you’ve got 6 ply radials on it now. You should always run radial trailer tires with ST before the dimensions at the max pressure identified on the sidewall if you stay within the speed rating of the tire. Under inflation leads to excessive heat buildup and is harder on trailer tires than running them at their max psi cold.