Once again, the National Highway Traffic Safety Administration (NHTSA) has issued proposed regulations requiring vehicles to be equipped with tire pressure monitoring systems (TPMS), this time by 2007. Remember, however, the last time NHTSA issued a “final” TPMS ruling in June 2002, it was drummed down by safety advocates of every stripe, and the regs were eventually struck down by a federal court.

This time, the NHTSA proposal, handed down in mid-September, would require vehicles to have a TPMS system with a yellow dashboard light that would illuminate when any one of the vehicle’s four tires was underinflated by 25% or more. Spares of all types are excluded.

The phase-in period for TPMS compliance also changed. Fifty percent of all new passenger vehicles must be so equipped by September 2006, 90% by September 2007, and 100% thereafter.

Interestingly, NHTSA’s proposed rule would require a new TPMS when a vehicle is first sold but wouldn’t require the systems to work when the OE tires are later replaced. NHTSA also found that certain tires can disable some TPMSs. Tires with reinforced sidewalls, especially some run-flat models, rendered some TPMSs ineffective, NHTSA claimed. Oddly, an effective TPMS is said to be the cornerstone of any workable run-flat tire system.

According to NHTSA’s proposal, the dashboard “telltale” icon would illuminate when a tire reaches “significant underinflation.” The yellow icon remains illuminated until the pressure in the offending tire is corrected. And NHTSA said TPMSs cannot be disabled by the car owner.

In reaction to the proposal, the RMA said that a 25% drop in a tire’s recommended pressure may be insufficient to carry a fully-loaded vehicle safely. In its new proposal, NHTSA turned down RMA’s request that tire inflation pressures allow for “reserve pressure” in a tire, sufficient to handle a vehicle safely when fully loaded. If the recommended inflation pressure for a tire is set at the minimum by carmakers, RMA said, a 25% drop would mean that the tires could be damaged because they don’t have enough air to carry the load. Motorists could be driving for thousands of miles on tires that are appreciably underinflated but not enough to turn on the warning light, said RMA.

TIA also has serious concerns. “A car with a gross axle weight rating of 1,885 pounds on the front axle has a recommended cold inflation pressure of 32 psi, which can support 1,058 pounds per tire. If the 25% threshold is used, the resulting air pressure is 24 psi, so the carrying capacity of the tire will be less than 900 pounds per tire, thereby creating a potentially overloaded assembly,” TIA said.

Some consumer safety groups have also weighed in against the proposal. There’s a lot arguing and jostling to be done yet. Don’t expect to see a final rule until mid-2005 at the earliest.