# eli5: Why can you only learn your proper tire pressure from the car and not from the tires themselves. Wouldn’t different kinds of tires need different pressures on the same car?

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Same tires, different cars = different recommended tire pressure

Different tires, same car = same recommended tire pressure

This doesn’t make sense to me. Why is this?

In: Engineering

Ummmm…. car manuals give you the tire pressure for the tires.

They base off cars due to weight etc

Truly, any non-OEM tire may require a different pressure. It is generally safe to stick with what is on the car’s door jamb, but different compounds, sidewall types, load ratings can effect the “best” tire pressure. Luckily, passenger cars are forgiving here so 30-35 psi (sorry, freedom units) is just fine for 90% of these vehicles. If you did change wheel size and tire aspect ratio, you’d want to experiment by feel and possibly a “chalk test”. You may even want different alignment settings.

Recommended tire pressure on a car is based off the car manufacturers recommended tire pressure for optimal driving in terms of handling, efficiency, and safety.

That depends on many factors of the car itself, its weight, wheelbase, length, height, etc.

The tires are able to work under many different pressures without breaking, so manufactures optimize pressure to the car not the tire.

Different cars weigh different amounts. That makes them require a different PSI in the tires to maintain a proper amount of traction.

Have you ever sat on an exercise ball and noticed how much it squished down? Then think about a kid sitting on the same ball, and how it’ll squish less? The ball is the same PSI in both scenarios, but it is holding more weight when you sit on it. This means that it gets squished into the ground more.

The same is true with tires — more weight = more squish. You want a certain amount of squish so that you have a certain amount of the tire in contact with the ground. You could have two cars that use the same tires, but one weighs 2000lbs and one weighs 2500lbs. If you use the PSI for the 2500lb car on the 2000lb car, your tires might not have enough contact area to maintain a good grip, and you’ll lose traction while driving.

To try to simplify this discussion away from the technical aspects of tires, suspensions, and cars; instead of thinking of the tires as balloons to be inflated to a particular size, think of them instead as mattresses to be adjusted to the different people lying on them.