As safe and responsible car owners we should all know that driving your car on tires which are either under or over inflated is not a wise move. It stands to reason that any temperature fluctuation can affect the air pressure inside our tires, so it is a good idea to understand the basic mechanical principles involved in this situation.

Air is a gas, which typically expands when heated. Tires heat up as we drive on them due primarily to friction, so to get an accurate measurement of your tire pressure you should check them when cold before driving. After driving, for at least 15 or 20 minutes, check them again and you should see at least a pound or two increase in pressure. This increase will be more radical if you are driving on an under-inflated tire. The tire is flexing a great deal in order to roll, and this flexing causes the rubber to heat up. Even a normally inflated tire will be warmer after being driven in colder weather.

 

image of tires

If you live in Florida where your average temperature is 70 or 80 degrees and you drive your car to Alaska, you may be surprised that your tire pressure reads a lot lower when you stop for gas up there. First principle is simple, air is compressible. When you compress a gas it tends to react more quickly to temperature changes than the same gas at atmospheric pressure.

If your tire is filled to the proper specification pressure at 35 degrees Fahrenheit, and your weather changes and it is 55 degrees outside, you will only see a marginal difference of about 2 pounds. Increase the temperature to 95 degrees from 35 and the pressure rise of can be dramatic at a full six pounds. Bear in mind not all tires have the same area inside them and they are not all operated at the same pressure either. When the same size tire is inflated to a higher pressure, it will react differently when the temperature rises. When your tires are normally at 55 psi versus 35 psi there is a lot less rolling resistance. This can increase if your tires are not inflated to the maximum specified pressure rating.

Tire manufacturers suggest that for every 10 degree increase or decrease in ambient temperature, a tire will show a 1 psi change. So if you have a deviation of 50 degrees that could be a 5 psi change from one extreme temp to the other. On a tire that is set for 35 psi this is a much more significant difference than a tire at 80 psi.

Typically we do not have to check and re-check our tire pressure, but it is important to keep an eye on them all year round and make adjustments to your tire pressure at the change of season. Many newer cars have tire pressure monitoring systems that help the operator keep closer tabs on the tires. This is a really neat way to make sure you are running the tires at the safest and most economical pressure setting.

Always make sure to check the manufacturer specification label on the car you are working on, which is typically a sticker on one of the door jambs. This pressure number should be very similar to the one molded into the sidewall of the tire. When in doubt, you can check with the auto manufacturer or the tire dealer as they maintain cross reference databases to ensure that they are offering the correct tires for every car and truck on the road.

Even if you cannot do the work yourself, stop at a tire shop or a full service gas station and have your tires checked periodically.