It's Getting Cold, So Time to Check Tire Pressure

The fall and spring are the most important times to check the pressure of your vehicle's tires. These are the times when they fall and rise, respectively, the most. This assumes you have no leaks in your tires. To understand why pressure changes we need to look at the ideal gas law:

PV = nRT

• P is the pressure, which I'll measure in psi (pounds per square inch). As an aside, we measure tire pressure in comparison to the ambient air pressure which is about 15 psi. Therefore the absolute pressure for a tire at 30 psi is actually 45 psi. Luckily we don't need to know this to maintain our cars.
• V is the volume. Car tires have stiff casings (carcass) which allows us to assume that volume is constant within a reasonable range centred on the recommended pressure.
• n is the number of air molecules. If our tires aren't leaking the number of molecules has not measurably changed since the last time the pressure was checked.
• R is a constant that is unique to every gas. Since we're all using air to fill our tires rather than exotic alternatives like carbon dioxide, nitrogen or helium, it is truly constant for our purposes.
• T is the temperature of the air. This must be measured from absolute zero (-273 C), so we have to convert the air temperature from Celsius to Kelvin. For example, 20 C is 293 K. You'll have to pretend there's a degree symbol (little 'o' at the upper right of the numbers) since I'm going to be lazy and not figure out how to do that in HTML.

Now we can simplify the above by using our assumptions. What we do is identify and eliminate all the constants since all we care about is the change in pressure due to temperature. This gives us the far simpler formula: P = T. More correctly this should say that P is proportional to T, not equal, though that's a mathematical nicety we can safely ignore.

What this formula states is that the tire pressure changes the same amount as the absolute temperature. This is just what we are looking for to understand why our tire pressure is lower in October than it was when we last checked in August. Let's say the tire pressure was 30 psi when the ambient temperature was 22 C. Here's what we get as the temperature drops:

• 10 C: 28.8 psi (-4%)
  
• 0 C: 27.8 psi (-7%)
  
• -10 C: 26.7 psi (-11%)
  
• -20 C: 25.7 psi (-14%)
  

These are large changes. A change of even 2 psi will degrade performance over bumps and holes and when cornering, and tire wear is accelerated. With lower pressures, efficiency drops, which raises tire temperature at high speeds, which causes increased wear and susceptibility to sudden failure.

The opposite changes occur in the spring as tire pressure increases with the warmer temperature. Of course if you swap between summer and snow tires this effect is avoided somewhat, however you are not home free.

Many people rarely if ever check tire pressure at all, preferring to leave it to the mechanics or oil jockeys during regular oil changes or service visits. If you've ever measured the tire pressure after these visits you will quickly learn you are far better off to take care of it yourself; all you need are an inexpensive barrel pump and a good pressure gauge. I do after every servicing and have found my 30 psi tires filled with anywhere from 25 to 40 psi. Modern tires are forgiving although handling and tire life are impacted.