Physics factoid:
The area of a tire in contact with the ground multiplied by the PSI of the tire gives the total weight the tire is supporting. There is a neat physics experiment where a wheel on a vehicle is jacked off the ground, the bottom of the tread is painted and a cardboard sheet is placed under the tire. The jack is carefully released so that the wheel does not roll but merely contacts the cardboard as it takes the weight transferred from the axle. The PSI of the tire is measured as accurately as possible, with interpolation. The wheel is jacked back up and the cardboard removed and the area of the paint mark on the cardboard is measured. The weight the wheel is supporting is computed. Doing all four wheels and adding the results gives the weight of the vehicle.
Running with too low an air pressure causes the tire to flatten because with the lower PSI the area in contact with the ground has to be greater to support the same weight. This causes the tire to flex through a more acute angle, decreasing the life of the tread. Running with too high an air pressure puts less tread on the ground and results in less flexing BUT it also increases wear in the center of the tread, causing it to wear faster than it normally would if the PSI indicated on the side wall of the tire were used.
Weighing cars with a pressure gauge, tire jack, cardboard and paint is fun. At least the boys think so!
