Yesterday while I rotated my tires, I decided to check the pressure while I had the tires off the vehicle. I inflated the tires to the usual 35 psi and installed them on the car. This morning I decided to check the pressure again (using my trusty digital Accutire gauge) and all 4 tires were at 33.5 or 34 psi. I'm going to rule out the possibility that all 4 tires leaked out essentially exactly the same amount of air as the odds of that occurring are astronomical. I'm thinking the difference is due to checking them while off the car vs on. However, if anything you'd think the pressure would read higher when the tires are on the vehicle because of the added weight of the vehicle. It doesn't make sense to me that it would be the other way around. Can anyone explain this?