At some point, almost everyone has seen the tire pressure monitoring system (TPMS) warning light appear on the dashboard. Its purpose is to warn you that at least one or more tires is significantly under-inflated, possibly creating unsafe driving conditions. The tire pressure readings are provided by pressure-sensing transmitters mounted inside each tire and sent to a central computer (ECU) for display on the dashboard. The problem is that the warning light on the instrument panel does not come on until there is a 25-percent drop in tire pressure. By the time the warning light is displayed, you and the passengers are in a very unsafe vehicle.
Let’s look at a real-world scenario: For its 2015 Suburban, Chevrolet recommends a tire pressure of 35 PSI. A decrease of 25 percent in PSI, triggering the warning indicator, would equate to a pressure of slightly more than 26 PSI – and that’s a very soft tire. The recommended tire pressure for the Mercedes S500 is 29 PSI. You wouldn’t see a warning light on the dashboard until one or more tires was down to less than 22 PSI.
If you are a VDI graduate, you know how much tire pressure affects a vehicle’s ability to corner, drive out of an emergency, and stop. In these scenarios, the driver would lose 15 to 20 percent of the vehicle’s emergency capability. That means if the vehicle/driver combination could escape an emergency at 40 MPH, they could not avoid the same scenario at 32 MPH, if the TPMS light is on.
Under-inflation also prematurely wears out tires and increases fuel consumption. A single pound of under-inflation takes 500 miles off of a tire’s life. Most tires only last 70 percent of their design life, thanks to under-inflation.
The answer? Don’t rely on the TPMS system to monitor your tires. Most luxury vehicles display individual tire pressures. Before picking up the boss, look at the individual tire pressures every day to ensure there are no surprises. It may be my age showing, but I also use that old-fashioned device called a tire-pressure gauge. Know the recommended vehicle tire pressures, loaded and unloaded.
Additional Information on TPMS
There are two types of TPMS, indirect and direct
- Indirect – An indirect TPMS doesn’t actually measure tire pressure. It relies on wheel-speed sensors that the antilock brake system uses. These sensors monitor how fast your tires are rotating and send signals to the computer that turn on the indicator light when something in the rotation seems wrong.
- Direct – Direct TPMS uses pressure-monitoring sensors within each tire that monitor specific pressure levels. The direct tire pressure-monitoring system wirelessly sends all of this data to a centralized control module where it’s analyzed, interpreted, and, if tire pressure is lower than it should be, transmitted directly to your dashboard where the indicator light illuminates.
TPMS is controlled by the government.
Federal Motor Vehicle Safety Standard 138 (FMVSS 138), requires original car manufacturers to equip all new vehicles with monitoring of tire pressure in all four tires.
This is the federal standard:
- A TPMS system operates when the vehicle ignition is on and warns when tires are underinflated by 25 percent or more.
- A TPMS system alerts the driver when there is a system malfunction.
- A TPMS warning light stays on until the tire is inflated to the proper pressure or the system malfunction is corrected.
- A bulb check of the warning light on the instrument panel occurs whenever the ignition is turned on.
According the FMVSS, the warning light comes on when the tire pressure in one tire is under-inflated by 25 percent.