# Tire pressure with tires on vs off the car

Today, I changed to summer tires on my Toyota RAV4 hybrid. Usually I check the air pressure after changing the tires (well, wheels, actually) with the tires on the car.

However, today I instead decided to check the air pressure of all tires before I installed them on the car.

I observed that all tires had just 2.2 bar, and I pumped them up to 2.4 bar to allow for some amount of air loss during the season (the recommended pressure is 2.3 bar, it's specified for cold tires). However, I after doing that, I became slightly concerned that having the car's weight on the tires could increase the air pressure, and thereby the air pressure could perhaps be too high.

Is my concern valid? Does tire pressure increase when the tires are installed to a car and the car's weight is on the tires therefore?

I at least know that temperature has a measurable effect on air pressure. But does weight load on the tires do the same?

• I'm sure there is some slight difference but you may not be able to measure it,
– Moab
Apr 15, 2018 at 12:46

The air pressure in tires are affected by temperature, which is why one measures and adds air when the tires are cold, undriven. When inflated, the shape of the tire will change slightly, as it expands. I'm sure many have seen a very low tire lift from the ground as the proper pressure is reached.

The contact patch of the tire to the ground is directly related to the weight of the vehicle and the air pressure. Let's say, for example, that the vehicle weighs 2,000 pounds and has one wheel and tire. The tire is filled with air to a pressure of 100 pounds per square inch.

This means that the contact patch of the tire with the ground will be 20 square inches.

If you lift the vehicle from the ground, the air pressure does not change within the tire, but the contact patch does. The shape of the tire then changes to allow for the release of weight.

If you add more weight to a vehicle without changing the air pressure, the contact patch will increase proportionally.

An article at http://vehicledynamics.com/the-tire-contact-patch/ does not reference directly the weight versus shape change but does refer to the effect.

In the world of bicycles, thin high pressure tires will have a contact patch that is distorted in the same direction as the bicycle travel. This indicates a greater area of distortion of the rubber as the bicycle moves. Energy is expended to distort the rubber.

A wider tire of the same pressure will have a contact patch distributed more across the width of the tire, but it will have the same contact area. With less contact along the line of travel, there is less distortion taking energy from the rider. Unrelated to this discussion is that wider tires have greater air drag.

There is no change to air pressure of a tire, on or off the vehicle, all other aspects remaining equal.