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

The optimal tire pressure depends on two things

1. Contact area of the tire – this is determined by the tire’s dimensions, width mainly

2. Weight of the vehicle

The tire pressure times the contact area will equal the weight of the car. If you under inflate the tires then the sides sag and more makes contact with the road, and if you over inflate the tires then it gets pointier and less makes contact with the road.

The tire only knows the first information, the car maker knows both because they specified the width of the tires that your car should be using. There could be tire pressure changes between cars of the same model but different trim levels if the 4WD ultra premium version adds another couple hundred pounds to the weight it could add 1 psi to the recommended tire pressure.

The specified tire pressure tries to keep the tires at their ideal shape for even wear, good grip (but not too much), snappy handling (but not too snappy), and acceptable noise. The tire could come with a charge of PSI by vehicle weight but its far more reliable to just always put it on the car