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Under pressure to keep tire warning light off

Dear Car Talk:

My question has to do with the air pressure in my tires.

In the winter, the low tire pressure warning light in my wife’s car tends to come on when it gets very cold. As I’m sure you’re well aware, this is due to the impact of the cold on the density of the air in the tire.

I live in Colorado, and it can be 14 degrees in the morning, and in the 60s by the afternoon. When I check the pressure early, it’s naturally low. When I check it in the afternoon, it’s back up to normal.

My question is this: What pressure should I set the tires at so that I can assure my wife that she is safe to drive, and will also prevent her sensor from sending her into a panic, and me having to check her air pressure to convince her that she does not have a flat tire?

Thanks. My frozen fingers thank you as well. — Dave

Good question, Dave.

Keep in mind that, generally speaking, tire pressure that’s too low is more dangerous than tire pressure that’s too high. So the answer is to fill the tires so that they’re at the correct pressure at the colder part of the day.

We know that tire pressure drops about one pound-per-square-inch (PSI) for every 10 degrees the temperature drops. So, if your recommended tire pressure is 30 PSI, and you set them at 30 PSI in the afternoon when it’s 65 degrees, by the next morning, when it’s 50 degrees colder, your tire pressure will be 25 PSI.

That’s more than 10% below the recommended pressure, so it’ll set off your tire pressure warning light, panicking your wife, and setting you on a path to frostbite.

So, instead, set the tires at 30 PSI in the morning. By the afternoon, the tire pressure will be 35 PSI, and that will be of absolutely no consequence in terms of safety.

If your wife is very sensitive, she may notice that the ride is slightly firmer. And if she’s an engineer, she may notice that she gets better gas mileage in the afternoon with her firmer tires than she does in the morning. Realistically, she probably won’t notice either (and neither would any of us).

As long as you stay below the tire’s maximum safe pressure (which is printed on the sidewall of every tire, and is considerably higher than the recommended pressure), your wife will be perfectly safe. At 35 PSI on a passenger car tire, you shouldn’t be anywhere near the tire’s maximum allowable pressure.

So in circumstances like yours, where the temperature varies wildly, set the correct pressure during the colder part of the cycle, Dave. Then you can both live like normal adults and stop thinking about your tires.

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