If you take a container of air and double the amount of air in it, the pressure will double also, assuming that you hold the temperature and volume constant.
OK, assume that you measure the pressure in your tire with a standard pressure gauge and it reads 32 pounds per square inch (psi). You now double the amount of air in the tire and assuming that the tire does not expand in any way and the temperature is held constant, what is the new tire pressure as read by your gauge?
I think he was getting at the point that the tire gauge measures not the pressure in the tire, but rather the difference between that pressure and the ambient air pressure, which is a little less than 15psi. So if the gauge reads 32, and using 15 as the ambient pressure, the pressure in the tire is 47psi. Double the air without changing volume or temp, and the pressure in the tire will double to 94psi causing the gauge to read 79psi.
