As discussed in some of our other articles on inflation pressure, tires must be properly inflated to achieve maximum handling, traction, and durability as designed by the tire manufacturer. It is the air pressure that supports the weight of your vehicle, not the tire itself. Tire pressure should be monitored frequently for safe driving and optimal tire performance.
Tire pressure can change with fluctuations in temperature. So, how should you correctly monitor tire pressure as the outside temperature changes?
First, it's important to remember that gas expands when heated and contracts when the temperature declines. In North America, the daily temperatures rise and fall between day and night, as well as seasonally. As the days get shorter and colder during fall and winter, it's especially important to check your tire pressure.
Second, it's important to know that the recommended tire pressure for your vehicle (as specified in the owner's manual and the tire placard for the vehicle) are both based on cold inflation pressure. This means that the tire pressure should be checked in the morning before the tire has been run, before the ambient temperature rises during the day, and before the tire is exposed to direct sunlight.
A good estimate to use when comparing tire pressure to air temperature is for every 10 degrees F, tire pressure will adjust by 1 psi. For example, if the outside air temperature increases 10 degrees, the tire pressure will increase by 1 psi. Conversely, if the air temperature falls 10 degrees, the tire pressure will decrease by 1 psi.
In most parts of North America, the difference between average summer temperatures and average winter temperatures is about 50 degrees F. This means that your tires will fluctuate approximately 5 psi (assuming no other air loss) between the coldest and warmest times of the year. A drop of 5 psi during colder months will affect traction, handling, and durability. This is why it's important to remember to check your inflation pressure, especially during colder times of the year.
In most parts of North America, the average daily air temperature fluctuates by approximately 20 degrees F. Using our rule-of-thumb formula from above, this means that the inflation pressure can fluctuate by approximately 2 psi during the day. This fluctuation can be even more pronounced if the tire is subject to direct sunlight.
In addition to changes from sunlight, the tires temperature is also affected by driving. Tire pressures can increase by up to 5 psi in the first 20-30 minutes of driving, before finally stabilizing.
If you live in a cold climate and have a heated garage, the opposite problem can occur. As soon as you leave the confines of your heated garage and go out into the colder weather, your tires can lose inflation pressure. Again, for every 10 degrees F dropped, the tires will lose approximately 1 psi each.
In all three scenarios, the same problem arises, regardless if you check the air pressure when the tires are warm from driving, when there are ambient temperatures, or if the tires are in direct sunlight. If the inflation pressure is correct when a tire is warm, then it will likely be anywhere from 2-5 psi under-inflated when it is cold.
Bottom line -- you should always check your tire inflation pressure when your tires are cold, and inflate them to the recommended pressure at that time.