By Eric Johnson
In the past 5 years there’s been a large increase in RVers who monitor their tire pressure with a tire pressure monitoring system (TPMS). When you have a TPMS and you can see your tire pressures in real time you start to realize very quickly that tire pressures can vary over a 24-hour period.
Let’s say it is travel day and you check your tire pressure at 8 a.m. and the ambient temperature at the RV park is 70° F. The pressure measured using your TPMS or calibrated gauge is exactly 100 psi. For this example, let’s say that 100 psi is the correct tire pressure that you should be running in your tires.
Now you are on the road and within a few minutes you are on the interstate traveling at 65 mph. Within five minutes of leaving the RV park, you notice that your TPMS is showing your tires at 107 psi. You keep driving and within 20 minutes your tires are now reading 112 psi while the outside temperature is still 70 degrees. Thirty minutes later the tire pressure is still 112 psi, but as the outside temperature rises, you notice even more rise in PSI and now you are up to 118 psi.
Interestingly, the side of the RV that the sun is shining on has pressures that are a bit higher than the side without the sun shining on it. At this point your tires have reached a steady state condition, but will they stay at this pressure assuming the outside temperature remains the same? We have already established that the outside temperature will affect your tire pressure, but what else?
The road surface you are traveling on will have an impact on the tire pressure as well; for instance, the difference between concrete and asphalt. The surface temperature of asphalt is usually hotter than concrete and may increase the tire pressure another one or two psi.
VEHICLE SPEED CAN INCREASE the tire pressure even further. Running at 75 mph versus 65 mph will generate even more heat and the tire pressure will rise even more. If you have dual tires, then you may also notice that your inside duals can run a little higher on the pressure side. This is because the inside duals have less air flow and therefore run a little hotter, and that increase in temperature can have a small effect on the pressure.
So, the big question here is should you be concerned that your tire pressure increases 15%, 20% or even 25% from your cold tire pressure? The short answer is NO. Tires are designed to take all these load, speed, and temperature variables into account when a commercial radial truck tire is designed, developed, and tested.
THIS IS THE REAL WORLD and is why all tire companies clearly state in their literature to never check a hot tire for pressure because you will think that the tire is overinflated when the air pressure is exactly where it should be. Don’t take air out of a hot tire. A truck tire can take four to six hours to revert to its original pressure. You simply do not know where in the cycle you are checking the tire pressure. The recommendation is to check your tire pressures first thing in the morning after the tire has cooled down overnight.
We have talked about tire pressure increasing, but what about tire pressure decreasing? Cold weather can create a different challenge as it relates to tire pressure. As an example, a tire that has cooled down after running all day and measures 100 psi at 70° F will lose pressure if it sits out overnight and the temperature drops to 20° F. When the tire is checked in the morning, you will find only 90 psi in the tire because when the temperature drops, so does the tire pressure. Every loss of 10° F equals a loss of two psi. Remember, tire inflation charts are based on ambient temperature of 68 degrees and do NOT include any inflation pressure build-up due to vehicle operation.
Another obvious reason your tire pressure may go down is because you have a leak. This low-pressure situation is among the most serious that you can encounter. Heat is a tire’s worst enemy. It is when a tire is running underinflated that an excessive amount of internal heat is generated due to the increased sidewall flexing and longer tire footprint (more rubber on the road). An underinflated tire is always much more serious to the tire’s health than a tire being a few psi over-inflated. Excessive heat will eventually lead to tire failure. Once a tire goes north of 200 degrees then the compounds in the tire start to break down and this is when a tire blowout can occur.
This is why we 100% strongly recommend every RVer run a quality TPMS. A good TPMS will monitor temperature and pressure. Since high temperature and low pressure are the two leading indicators of a future blowout, by monitoring these points of data you can be warned of problems to hopefully give you time to pull over and prevent the blowout. We recommend the TST Tire Pressure Monitoring System and have used this system for the past 5 years. Check them out if you do not already have one.