Dear Tom and Ray:
The car manuals all say to measure tire pressure when the tire is cold, but none defines the temperature (or range of temperatures) for "cold." What temperature defines a "cold tire"? -- Bill
RAY: Good question, Bill. There are two kinds of temperatures that affect tire pressure. There's the outside, ambient temperature, and then there's the temperature INSIDE the tire, which gets elevated when it's driven on. When manufacturers refer to "cold tires," they mean "tires that have not been driven on recently."
TOM: So, you can have a cold tire on a 100 degree day, and a cold tire on a 2 degree day. They're both considered cold tires as long as you haven't driven any significant distance on them in the past few hours (a few blocks to the gas station is fine).
RAY: Here's why the tires need to be cold to be measured. Let's say you go out in your pajamas and check your tire pressure first thing in the morning, after the car has been sitting still all night. It measures 30 psi. If you drive around for an hour, stop at a gas station and check the pressure again, it will be higher.
TOM: You'll also be standing at a gas station in your pajamas.
RAY: True. But that second reading is affected by the heat of the tires themselves. And since the pressure in a hot tire varies tremendously throughout the day, manufacturers calibrate their recommendations based on a cold tire. It's kind of like a "resting heartbeat."
TOM: Now, you might ask, can't the outside, ambient temperature affect tire pressure, too? Good question! Yes, it can, and it does. That's one reason it's important to check your tire pressure at least seasonally.
RAY: Tire pressure rises or falls about 1 psi for every 10 degrees of outside temperature. So, let's say you check your tire pressure (on a cold tire, of course) in July, when it's 90 degrees out. It's 30 psi. If you check it again in January, when it's 20 degrees, your tire pressure will be 23 psi, which is dangerously low (70 degree change = 7 psi change). So, you can't ignore the effect of the outside temperature.
TOM: And by the way, even if temperatures don't vary a lot where you live, it's still important to check your tire pressure regularly, because lots of tires do leak air slowly.
RAY: But by checking the tire pressure -- in every season -- when the tire is cold (like first thing in the morning, or when the car has been sitting for several hours), you eliminate the variable of road friction. That's all they mean by cold tires.