Why is the air (atmospheric) pressure of 14.7 lbs/in2 not included in the pressure gauge reading for a tire?

I made an error - Why is the atmospheric pressure of 14.7 lb/in2)not added to the pressure of the tire gauge when computing the weight of the vehicle?

You can only get an approximate weight of the vehicle by multiplying the tire gauge pressure by the area of the tires in contact with the road.

It is appropriate to use the gauge, rather than the absolute, pressure in such calculations because atmospheric pressure acts on the entire exterior of the car except the road contact area. This has the effect of creating a buoyant force that equals the reduction in weight that you get by using the gauge pressure in the weight calculation.

The pressure gauge reading for a tire does not include the atmospheric pressure of 14.7 lbs/in² because it is considered to be already present and acting on both the inside and outside of the tire.

When the tire is inflated, the pressure inside the tire increases above the atmospheric pressure. The pressure gauge is designed to measure this increase in pressure relative to atmospheric pressure. Therefore, the gauge reading only displays the additional pressure caused by the inflation of the tire, rather than the total pressure acting on it.

By not including the atmospheric pressure, it allows for easier and more accurate measurement of the pressure change inside the tire, which is more relevant for determining the proper inflation level and checking for any loss of pressure that might indicate a leak or other issue.

The air pressure of 14.7 pounds per square inch (psi) is not included in the pressure gauge reading for a tire because it represents atmospheric pressure, which is the pressure exerted by the Earth's atmosphere on all surfaces at sea level. When filling a tire with air, the pressure gauge measures only the additional pressure that is being applied to the tire, above and beyond the atmospheric pressure.

To understand this concept, let's consider a simple analogy. Imagine you have a glass of water filled up to the brim. The surface of the water is at the same level as the top edge of the glass. Now, if you were to pour more water into the glass, you would measure the additional height of the water above the brim, not the entire height from the bottom of the glass. Similarly, when measuring tire pressure, we are only interested in the additional pressure above the atmospheric pressure.

In practical terms, when you attach a pressure gauge to a tire, it measures the difference between the air pressure inside the tire and the atmospheric pressure outside, providing a reading that reflects only the extra pressure in the tire. For example, if the pressure gauge reads 32 psi, it means that the air pressure inside the tire is 32 psi above the atmospheric pressure, which is approximately 14.7 psi. Therefore, the actual absolute pressure in the tire would be 32 psi + 14.7 psi = 46.7 psi.

By excluding the atmospheric pressure from the reading, it makes it easier for people to understand and work with relative pressure values, as measuring the additional pressure allows for more accurate monitoring of tire pressure and inflation.