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?

Question

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

Answer 1

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.