Dear Car Talk:

I have always thought that tires were, along with brakes, the first line of defense in a car. I know from a lot of biking that tire pressure has a big effect on rolling resistance and therefore on fuel economy.

Whenever I take my car for an oil change, they inflate the tires to 35 psi, even though the plate inside the driver's door says to use 33 psi. I have to always ask them to correct it.

I'm guessing a lot of people overinflate their tires to improve mileage. But I assume that the engineers who designed the car gave careful consideration to safety, comfort and economy when deciding on the tires and the correct pressure.

Of course, where I live, in Minnesota, we can have temperatures of 20 one day and 30 below zero the next, so maintaining the correct pressure isn't always easy. That being said, what is your opinion?

Best to set the correct pressure frequently to keep the correct amount of rubber on the road, or overinflate for economy? Does it matter that much? -- Barney



Obviously, the best thing to do is to check your tire pressure three times a day to makes sure it's always exactly what the manufacturer calls for. But nobody does that. Nor would we recommend it. "Sorry, I can't make your college graduation, dear. I'm scheduled to check my tire inflation during the ceremony."

So given that people don't check their tires frequently and that temperatures vary, we recommend overinflating your tires by a little bit. It's always more dangerous to underinflate tires than to overinflate them -- within a reasonable range.

As a rule, you never want to drive on tires that are more than 10 percent below their recommended pressure (that's typically when your tire pressure warning light will come on, if your car has one). Underinflation can cause heat to build up and tread to separate, causing a blowout (see the Ford-Firestone controversy, February 2000).

So if your car calls for 33 psi, you never want to let the tires go below about 30 psi. But tire experts say running them at 35 or even 38 or 40 is not going to endanger you or cause any meaningful degradation in braking or handling. At worst, you'd have a stiffer ride, and have a few welts on your head from hitting that pesky dome light.

If you live in an area where temperatures vary widely, you're better off overinflating by a bit. Tire pressure changes about 1 psi for every 10 degree change in temperature.

So if your temperature goes from 20 one day to 30 below zero the next, your tire pressure would drop 5 psi. If you started at 33 psi, you'd drop to 28 psi, which is too low. Whereas if you started at 35, you'd drop to 30 psi and still be fine.

If you live where the temperature is stable, use the manufacturer's recommended tire pressure, or a bit more. And check your pressure once a month or so to account for slow leaks and the change of seasons. Or just wait for the warning light to come on, if you have one.

And if you live where the temperature is all over the place day to day, then overinflate by a few psi to be safe.