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Does a tire's air pressure vary depending on whether or...

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Dear Tom and Ray:



Does a tire's air pressure vary depending on whether or not it is supporting the vehicle's weight? I'm wondering if my spare tire, which is supporting nothing, should be inflated to the same psi as the other tires, which are bearing the weight of the vehicle. -- LeeAnn

TOM: Great question, LeeAnn. And, like most great questions, we don't know the answer. So we consulted with Paul Murky at Murky Research, and he devised an experiment for us.

RAY: On Paul's advice, we took my car into the garage and took off two of the wheels.

TOM: Then we measured the tire pressure on those "removed" wheels with a good -- but not scientific-quality -- dial gauge, and both tires had 30 pounds of pressure in them.

RAY: We then put the tires back on the car, lowered the car to the ground and measured the pressure again. And the pressure in each tire was ...

TOM: 30 pounds! Or at least as close to 30 pounds as we could see with the naked eye and our gauge.

RAY: So the answer is that you should inflate your spare tire to the same pressure that you inflate the rest of your tires. Whether it's on or off the car appears to make no significant difference.

TOM: And I would have to guess that the reason it makes no difference is because properly designed tires maintain the same exact volume inside, despite the fact that they deform under the weight of the car.

RAY: You'll notice that the tires change shape under the weight of the car, flattening a bit on the bottom but widening a bit on the sides. Kind of like my brother did as he got older.

TOM: But because they've been designed to handle the weight of the car and have enough sidewall strength, the volume inside the tires stays the same, and thus the pressure stays the same.

RAY: Keep in mind that this only works if the tires are designed to handle the weight of your car. If you put a bicycle tire on your car, for example, the weight of the car WOULD crush the tire and decrease the interior volume. In that case, the pressure would shoot up and cause the tire to explode. Not to mention what would happen to those spindly little spokes!

TOM: So you want to use the tire sizes and ratings recommended by your manufacturer, and abide by the weight and cargo limits for your vehicle as well. And then check the pressure -- including the spare -- once a month, if possible, and at the very least, once a season, and you'll be all set, LeeAnn.
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