Trucking professionals have seen the word “cold” molded on a tire’s sidewalls next to the maximum load and corresponding inflation pressure for decades. It basically means that tire pressures should be checked when air temperatures and tire temperatures are the coolest. During the summer months, when the average low temperature is between 60 and 80 deg. F across most of the country, cold inflation pressures are probably the most consistent throughout a fleet. But in the fall, ambient temperatures can have a much wider spread, which can lead to problems.
For example, at the time of this article, the lowest high temperature is 38 deg. F in the Northern Rocky Mountains; the highest high temperature is 91 deg. F at the tip of Texas and Southern Arizona. A truck that leaves in the North with tires inflated to 100 psi will have inflation pressures that read around 110 psi when it reaches the South, and “cool” off to an ambient temperature that is about 50 deg. warmer. Likewise, the truck in the South that starts at 100 psi in 90 deg. F weather will read about 90 psi when the temperatures reach around 40 deg. in the North.
The general rule is that inflation pressure changes by 2 psi for every 10 deg. F change in temperature. When the tire gets hotter, the inflation pressure goes up because the air molecules increase in volume; when the tire cools off, the temperature goes down because the volume of air decreases. It’s called Boyle’s Law and it states that pressure and volume are inversely proportional so in a contained pressure vessel like a tire, any changes in volume will be reflected in pressure.
This is an important time of year to check inflation pressures. Tires that have been getting through the summer months with partial underinflation are soon going to be exposed to cooler fall mornings. At 95 psi, a 295/75R22.5 can carry 5,070 lbs. in a dual application, so the tires can support the full 20,000 lbs. allowed on a single axle in most states. In the previous South to North example, the carrying capacity drops to 4,690 lbs. when the inflation pressure falls to 85 psi. This means a fully loaded vehicle will be overloaded even though the tires were able to carry the weight the previous day when the temperatures were warmer.
It’s also important to note that in the same South to North example, the 95 psi tire may still read close to 95 psi when it first reaches the colder temperatures. While the increased heat from the flexing sidewalls causes the temperature and inflation pressure to rise, the cooler air temperatures offset those increases to some degree. In the North, the technician or driver who sees 100 psi on the gauge at the end of the day is practically guaranteed to see much lower inflation pressures the following morning when the temperature difference can be 50 deg. F or more.
Maintaining the proper inflation pressure is vital for tire performance and fuel economy. Underinflated tires are more prone to failure and irregular wear, create unnecessary rolling resistance, and can quickly have a negative impact on the bottom line. Nobody wants to crawl out of bed on a chilly fall morning to check tire pressures, but the consequences of ignoring the change in temperature can be expensive. And while it should be done on a regular basis regardless of time of year, the process of installing “winter air” is more important than ever with tire and fuel prices near record-high levels.