If you have purchased a new car in the last few years, I hope you noticed the yellow horseshoe light with the exclamation point that lights up on the dashboard during the bulb check. This indicator informs the driver that the vehicle has a tire pressure monitoring system (TPMS) that is designed to signal when any tire (or combination of tires) is 25% below the inflation pressure listed on the placard. It's an early warning sign alerting the driver that a tire is losing air and has crossed the dangerous threshold towards imminent failure so immediate attention is necessary.
The trucking industry has talked about TPMS for years, and I have several columns encouraging fleets to explore the potential savings that accompanies it. There's no question that TPMS has benefits, but I'm also sensitive to the fact that margins in trucking are razor thin right now so the capital investment necessary to implement TPMS is restrictive for most fleets.
The good news is that every fleet has a TPMS system that is almost foolproof — the driver. Drivers are required to perform a pre-trip and post-trip inspection that includes the tires. In fact, according to Federal Motor Carrier Safety Administration (FMCSA) Regulation 393.75 (h) Tire Inflation Pressure (1), “No motor vehicle shall be operated on a tire which has a cold inflation pressure less than that specified for the load being carried.”
There are two ways to check the inflation pressure in a truck tire. The first, and preferred method, is to place a calibrated air gauge on the end of the valve stem so the pressure can be read on the dial, readout or stick. The second is to use the old boot-o-meter, or the infamous tire thumper. I've done plenty of fleet maintenance seminars where I inflated four truck tires to different pressures and then asked drivers and technicians to find the flat without an air gauge. Some of them even went to retrieve their own tire thumpers. While I've never kept statistics, the majority of the students could not find a tire that was 5 or 10 psi below the other tires without an air gauge. To further demonstrate my point, I would ask the students to guess the actual inflation pressures that often produced pressure ranges of 20 to 40 psi.
Checking the inflation pressure of a tire with a boot or tire thumper is like checking the oil by putting a hand on the hood of an idling truck. It may find the obvious flat, but it will not consistently locate the tires that are below operating pressure. In fact, if all eight tires on a trailer were inflated to the same air pressure at some point in time and then never checked or inflated again, the tire thumper method will probably make the driver think the tires are properly inflated for years because there won't be much difference in the sound that the thumper makes when it hits the tires.
If the TPMS behind the wheel follows the federal regulations for inspecting truck tires as part of the inspections, an inflation gauge will be used on every tire before and after every trip. And while fleets should spend the extra money for inflate-through valve caps to make it easier, the driver is still required by law to make sure the pressure is sufficient for the load with something a little more reliable than a wood baton or sawed-off baseball bat.