|With the TPMS, one of these symbols will light up on the dashboard when any tire is 25% under-inflated.|
September 1 marks the start of required 100% compliance with the US federal mandate for all new vehicles of 10,000 pounds (4,536 kg) or less gross vehicle weight to have a four-tire pressure monitoring system (TPMS) that is capable of detecting 25% under-inflation and warning the driver.
The TPMS rule (FMVSS 138), crafted in 2002, began its phase-in in 2005. The rule is designed to address fuel economy and safety issues caused by under-inflated tires. Fuel efficiency is reduced by 1% for every 3 psi under inflation, according to the industry.
In terms of safety, dry and wet handling performance also suffers with under inflated tires. The types of crashes that under-inflated tires influence include skidding and/or loss of control, hydroplaning on a wet surface, and crashes from flat tires or blowouts.
The rule is technology-neutral, allowing any TPMS design that complies with the performance requirements. Performance testing is to be done with tires on vehicle at time of first retail sale.
There are two basic types of TPMS: Wheel-Speed Based (WSB, referred to as “indirect”) and Pressure-Sensor Based (PSB, referred to as “direct”).
Wheel-Speed Based TPMS infer tire pressures using the vehicle’s ABS hardware, specifically the wheel speed sensors, to measure tire-to-tire differences in rotational velocities. Pressure-Sensor Based TPMS directly measure tire pressures with pressure sensors mounted either in each tire or on each wheel.