Transportation Science

How do you measure tire pressure?
Answered by Discovery Channel
  • Discovery Channel

    Discovery Channel

  1. You use a tire pressure gauge to measure the air pressure inside a tire. First, you put the small sphere at the end of the pressure gauge onto the valve stem of your tire. Press on the valve stem to form a seal, releasing air from the tire into the gauge (the seal will prevent air from leaking out). WIth the seal created, a spring in the gauge pushes against the incoming pressurized air, and a calibrated rod in the gauge measures the force of this air.

    There are digital tire pressure gauges that offer a numeric readout, as well as more traditional gauges that use a pointer to indicate tire pressure. In either case, what you're looking for is a measurement of pounds per square inch, or psi. Every tire lists on its sidewall a maximum psi rating for the tire itself, and every auto manufacturer lists a recommended tire pressure setting for use on that particular model of car. You can check the owner's manual to find not only the proper psi setting but also the right size tire for your car. Another place where car manufacturers often list the recommended tire pressure is on a small metallic placard on the inside of one of the car's doors.

    One thing that can be easily overlooked when you're checking your tire pressure is the pressure of the all-important fifth wheel: your spare tire. If it's not a full-size tire and is instead a "donut," check its sidewall, because the psi recommendation will likely be different from a full-sized tire.

    Finally, if you're the very thorough type, a handy device you can stow in your car at all times is a portable air machine. There are more expensive variations, but you can usually find small, inexpensive units at your local auto parts store. They will run off of your car's in-dash power adapter, providing air in a jiffy, if you have a flat and realize you forgot to put air in the spare tire.

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