TPMSs are a safety requirement established by the NHTSA after the vehicle rollover deaths caused by Firestone tire failures.
Vehicles with dual rear wheels, because of the minimum risk of having two tires fail on one side did not require TPMS on the dual tires. Here is an excerpt from that ruling. Some of this specific ruling was also influenced by the cost to the transportation industry of maintaining the system:
Consistent with the Second Circuit#65533;s opinion, FMVSS No. 138 requires new passenger cars, multi-purpose passenger vehicles, trucks, and buses with a gross vehicle weight rating (GVWR) of 4,536 kg (10,000 pounds) or less, except those with dual wheels on an axle, to be equipped with a TPMS to alert the driver when one or more of the vehicle#65533;s tires, up to a total of all four tires, is significantly under-inflated. Specifically, the TPMS must warn the driver when the pressure in one or more of the vehicle#65533;s tires is 25 percent or more below the vehicle manufacturer#65533;s recommended cold inflation pressure, or a minimum level of pressure specified in the standard, whichever pressure is higher. (We note that in response to a petition for rulemaking by the Alliance of Automobile Manufacturers (Alliance) and that organization#65533;s subsequent, related comments on the NPRM, we have decided, as an interim measure, to modify our minimum activation pressure (MAP) values for some light truck tires under the standard. Once the agency conducts further safety research, we will either confirm or propose to modify these MAP requirements in response to that petition. )
You can read the entire document here:
http://www.nhtsa.gov/cars/rules/ruli...rule.6.html#II
DEPARTMENT OF TRANSPORTATION
National Highway Traffic Safety Administration
49 CFR Parts 571 and 585
[Docket No. NHTSA 2005-20586]
RIN 2127-AJ23
Federal Motor Vehicle Safety Standards; Tire Pressure Monitoring Systems;
Controls and Displays
Linear Mode
