If you check a tire pressure of a tire/wheel that is off of a car will it be less than if it was on a car (supporting the cars weight)?

Sorry for the dumb question but I have never had a second set of wheels/tires before. This is a new one for me.

 

It won't change a whole lot, probably not enough to measure.

If you want to do an experiment, you don't even have to remove the wheel from the car. Just jack the car up so the wheel is off the ground - there is no force on the tire then. Measure it "off the ground" and "on the ground" and see if you can detect a difference.

 

The difference is usually less than half a psi on compact cars...so unless
you have a digital pressure guage, you won't be able to tell that much.