just to be anal about the pressure rise, absolute should be used instead of gauge pressure to compute the percentage rise, so instead of 21% (9/42) its about 16% rise (9/57).
Agreed. It would be absolute for the gas equation, but I was just using the ratio of 1 psi for 10F rise to calculate it. When most rules of thumb for tire pressure rise say 5% pressure rise for best tire life and 10% pressure rise for best performance, they are using gauge pressure. 10% would be 40 to 44 psi and 5% would be 40 to 42 psi. That was the comparison I was making with the 20% number - twice the normal rise that most people would look for when trying to set cold pressure to a level that gets the tires to best operating temperature. I've never had one go up 9 psi after being run. And if I had gotten a 9 psi rise, I'd think my starting pressure was too low. But at 42 psi, that is not low. I'm still puzzled by the numbers.