That is not the norm for that size bias tyre but we don't do norm, I made that mistake in 1990 running 4psi on a 14x32 slick cos thats what everyone else ran, pushed it up to 6psi and it broke the Tach off the dash on the hit.....its what worked best for our 3800lb combo at Santa Pod, 1 more psi and it turned the tyre. My mate's 3150lb Dart 60'd best with 13psi on a 13x31 tyre, run as much as you can so you don't kill the tyre.