Dear Tom and Ray:

There has been a lot of publicity lately regarding the correct pressure (26 psi versus 30 psi) for certain types of tires on certain types of vehicles, which I won't mention by name. (Editor's note: We'll mention 'em. It's Firestone tires on Ford Explorers.) But nowhere have I seen anybody discuss the fact that tire pressure increases dramatically with heat. Obviously, 30 psi in your dealer's garage is something very different when it's 90 degrees outside and you're driving on a hot, Texas interstate where the asphalt is 150 or 200 degrees. Should you use the same "standard" tire pressure on a hot summer day in Texas as you would on a winter day in Minnesota, or should you lower the pressure in hot weather to compensate? -- Bill

RAY: You should definitely not lower the tire pressure, Bill. Our advice has always been to fill your tires to the manufacturer's recommended pressure, and keep it there. In fact, the reason you need to check your tires once a month or so is precisely because the pressure WILL go down when the outside temperature goes down, and you don't want to inadvertently drive with low tire pressure.

TOM: You're absolutely right that the pressure of a hot tire goes up, Bill. But on a non-defective tire that's been properly matched to its vehicle (and here's where we won't mention certain types of tires and certain types of vehicles), that rise in pressure has already been accounted for, and it should be safe to drive at highway speed on a hot Texas day.

RAY: If the tire is defective, improperly designed or really wasn't meant to handle the weight of the vehicle, then you can have tire failure under those conditions -- which ARE the most stressful conditions for tires.

TOM: But you should absolutely NOT lower the tire pressure to try to counteract the effect of the heat. It actually has the opposite effect.

RAY: Right. Lowering the tire pressure lets more of the tire's surface "squish down" onto the road. That bigger "contact patch" creates more friction, which creates more heat, which CAN cause the tire to fail. In fact, some people believe that Ford's relatively low recommendation of 26 psi for the Firestones contributed to those well-publicized tire failures. And in the past few weeks, Ford has upped its recommendation to 30 psi. Coincidence? You decide.