The air inside your tires is getting colder too, so it's shrinking. Even if air isn't leaving your tires, the pressure inside is going down because of contraction.
Your tires will lose about one to two pounds of pressure for every ten degrees of outside temperature, says Sinclair.
If you have a new car, it probably has a tire pressure monitoring system that turns on an amber dashboard warning light when the tire pressure falls too low. As with most "idiot lights," however, by the time that light comes on, your tires are already lower than you should ever let them get.
Likewise, don't wait until your tire "looks low." Tires often look low when they're not and vice-versa.
The air pressure in your tires should be checked in the morning before you've driven on them, advises Ibbotson, which is when they're at their coldest.
The recommended tire pressures in your owner's manual or stamped someplace in your car - usually inside the driver's door - are recommended for when the tire is cold, not after it's warmed up. (You should go by those numbers, not what it says on the tires, in case the car requires a certain pressure for proper ride and handling.)
