Tire pressure drops along with falling temperatures. Your car may sound the alarm this weekend.
As record cold moves into the Northeast, New Englanders will receive their first blast of truly cold air this winter season. Along with that cold air comes a host of car-related problems such as weak or dead batteries. However, the one system in your car that may work as designed and still give you a hassle is the Tire Pressure Monitoring System (TPMS). This federally-mandated safety system monitors your tires for signs of a puncture, but most systems also watch the pressure in all four tires. In most systems if the pressure drops below a set minimum the TPMS sensor will come on and alert you to the situation. Many drivers may misinterpret this as a flat tire warning.
The reason that your tires have less pressure is simple physics. There is no leak happening. The same amount of air will be inside your tires when the temperatures drop to -10F as was in them last week when it was 50F. However, the pressure will fall along with temperature. The formula that dictates this is PV=nRT, or more simply, PV/T= P2V2/T2. Temperature and pressure are inverse to one another in the formula. A couple of old-school scientist names Boyle and Dalton figure all this out back in the days when doctors were still applying leeches to sick people and blaming the “ethers” for maladies. We could crank out the exact number of pounds per square inch your tires will lose using the formula, but we car nuts know from experience it ends up being about one psi for every ten degrees. So if you started off with 30 psi last week, your pressure inside the tires may drop to around 24 psi by Sunday morning when temps bottom out. If your TPMS system does give you a warning, fill your tires and then reset the system. Your manual will have instructions on how to do that.
The solution to the problem is simple, add air to all of your tires. Every U.S. vehicle has the correct pressure settings printed on the drivers door jam. Usually, it is around 32 psi. Don’t go crazy and add more. When temperatures rise, so too will your pressure. Just set it at what the manufacturer recommends and then check it again in the spring to see if you can let a little out.