# Why do compact spare tires take a higher pressure?

It seems that a common arrangement with midsize cars in the US is that the regular tires are inflated to 35 psi, but the spare (which is smaller), is labeled for something much higher, like 60 psi. Why?

My hypothesis is that it has to do with the force that the air exherts on the vehicle. Large tire * low pressure = ideally rated force. Small tire * high pressure = same ideally rated force, or at least in the ballpark.

Is my basic physics correct? Is this why?

Yes, you can roughly tell how heavy a vehicle is by multiplying the contact patch area of all four tires by their inflation pressure, and add them together.

to handle the same weight, a smaller tire with a smaller ideal contact patch is going to need a much higher inflation pressure than a large tire.

The air pressure inside the tyre is equal to the pressure exerted by the tyre on the road. If the contact patch of the tyre is halved (roughly as in a space saver) then the pressure must be doubled to support the same amount of weight.

Isn’t the answer simpler than that? It seems a smaller diameter/lower volume tire needs more air so that when hitting a bump, it doesn’t deform to the point of hitting the rim, thus damaging the rim, tire, or both. ??

No it is the smaller contact area and needing higher pressure.

Is this why cars can print the recommended tire pressure on the door jamb without any knowledge of the type/construction of the tires (save recommended size)?

I think that’s mostly just that tires are fairly standardized, without all that much significant difference. If you had tires that were of significantly different width, the recommended pressure wouldn’t be what it lists on the door jamb (the compact spares here being an example of that, of course, as would wider-than-normal tires).

Right, I assume that the listed pressure is for the recommended/factory tire size only. I’ve always marveled that a car manufacturer can put one pressure whether I’m using bargain basement Sillyputty X-treames or uber-expensive Racing Technomarvels. I would have thought that construction factors (sidewall strength, materials, etc.) would vary between manufacturer. That’s why it was always baffling to me that they could list one pressure without any knowledge of what you’re putting on the car (again, save that the size is what they expect).