Tire Pressure Monitoring Systems

Section 13 of the Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act,
which Congress passed on November 1, 2000, directed NHTSA to conduct rulemaking actions to revise
and update the Federal Motor Vehicle Safety Standards (FMVSS) for tires, to improve labeling on tires,
and to require a system in new motor vehicles that warns the operator when a tire is significantly
underinflated. In response, in 2005 NHTSA published the final rule for FMVSS No. 138 which requires
that passenger cars, multipurpose passenger vehicles (MPVs), and trucks and buses with a gross vehicle
weight rating (GVWR) of 10,000 pounds or less be equipped with a TPMS that is capable of detecting 25
percent underinflation in any combination of tires. The rule specified a phase-in schedule from 2005
1 Rubber Manufacturers Association. “Tire and Auto Safety.” [Online] Available at
http://www.rma.org/tiresafety/auto_safety_facts.html, March 2001.
2 Final Regulatory Impact Analysis, Tire Pressure Monitoring System, FMVSS No. 138, March 2005, NHTSA, Docket
No. 2005-20586-2.
through 2007, with all vehicles manufactured on or after September 1, 2007, required to be equipped
with TPMS.
These systems must alert the driver to low tire pressure (pressure 25% or more below the
manufacturer’s recommended cold tire inflation pressure, which is typically located on the door jamb
label) through a dash displayed warning light. The display must activate within 20 minutes of
underinflated travel at speeds of 50-100 km/hr and must remain illuminated until the underinflation is
remedied. The system must also have a malfunction lamp in addition to a low pressure warning lamp
that alerts the driver if the vehicle’s TPMS is not functioning properly. Additional details of the
requirements of FMVSS No. 138 can be found in the Code of Federal Regulations.3
Two distinct TPMS technologies have been installed in production vehicles at various times since 2000:
Direct TPMS: Also known as pressure sensor based (PSB), direct systems use a physical pressure sensor
inside each tire that sends information to a central processing unit in the vehicle. Some direct systems
exceed the minimum requirements of the standard and display the pressure of each tire on the dash
allowing the driver to diagnose overinflation as well as underinflation. The system’s sensors are most
often located on the interior end of a tire’s valve stems. Therefore, pressure information must be
transmitted to the vehicle via a battery-powered radio frequency transmitter. Unlike indirect systems
that do not have sensors in each wheel, direct systems may require additional maintenance and repair
when pressure sensors malfunction due to harsh environmental conditions, depleted batteries or if they
are removed, replaced or otherwise affected during tire repair or replacement.
Indirect TPMS: Also known as wheel speed based (WSB), indirect systems use individual wheel rotation
speeds gathered from the anti-lock brake system (ABS) wheel speed sensors to detect relative
underinflation. The operating principle is that if a single wheel has a faster rotational speed than other
tires, then its radius or rolling circumference must be smaller and therefore the tire may be
underinflated. Because indirect systems produced before FMVSS No.138 were only capable of
diagnosing underinflation in relation to the other tires on the vehicle, these systems experienced
difficulty identifying underinflated tires if several tires were simultaneously underinflated. FMVSS No.
138 required that monitoring systems must be capable of detecting underinflation in ‘one or more of
the vehicles’ tires, up to a total of four tires’, and as a result most manufacturers began exclusively
installing direct systems after publication of the final rule. Fewer than three percent of the vehicles
observed by the TPMS Special Study were equipped with indirect systems.
Recently some manufacturers have begun to produce indirect systems that integrate other types of data
from the ABS and electronic stability control (ESC), along with information from other evolving
technology sensors, into the diagnostic model. At the time of publication there are two vehicle
manufacturers, Volkswagen and Audi, which utilize indirect systems in production vehicles. The first
vehicle with an indirect system that met the requirements of FMVSS No. 138 was the model year 2009
Audi A6. In 2010 and 2011 both Audi and Volkswagen began to include indirect systems on several
models including the Audi A3, A4, A5, A6, A8, Q5, and Q7and Volkswagen’s Golf, GTI, and Jetta. Post-
3 Code of Federal Regulations, Title 49, Part 571.138
FMVSS No.138 indirect systems may have many practical advantages over direct systems in terms of
long-term maintenance because there is no physical sensor installed on the wheel and the systems do
not require independent batteries. Although this survey did not collect sufficient data from these post-
FMVSS No. 138 indirect systems to evaluate their effectiveness, their performance may be similar to the
direct systems evaluated in this report (because they also comply with the requirements of FMVSS No.
Display Types: To meet the requirements of FMVSS No.138 a vehicle is required to have a warning lamp
that alerts a driver to severe underinflation as well as a malfunction lamp that indicates when the TPMS
is not functioning properly. In addition, some vehicles are equipped with tire specific warning lamps
that tell the driver which tire is triggering the underinflation warning. Other vehicles have tire specific
inflation pressure readouts that are capable of reporting the current pressure of each tire at any time in
addition to an underinflation warning lamp. These systems may alert drivers to less severe
underinflation that has not yet reached the 25% threshold needed to activate the warning lamp. They
may also prevent overinflation, which is not identified by other types of TPMS displays. Pressure
displays are only possible with direct TPMS. These display types are evaluated separately in Section 3.4
(underinflation) and Section 4.4 (overinflation) of this report to explore any possible differences in
Market Share: Figure 1 below shows an estimate of the percent of new vehicle models sold with TPMS
for model years 2000-2008 (TPMS became mandatory for all new vehicles manufactured on or after
September 1, 2007). These estimates are based onWard’s Automotive Yearbook data for 98 popular
vehicle models.
Figure 1: Estimated Percent of Vehicle Models Sold With TPMS
Model Year
Total TPMS
Direct Std
Direct Opt
Indirect Std
Indirect Opt
The steep rise in overall TPMS seen in 2007 is a result of the phase-in schedule specified by FMVSS No.
138, which required that at least 20 percent of all model year 2006 vehicles and 70 percent of all model
year 2007 vehicles be equipped with TPMS. The graph also shows that indirect systems were not very
common before the standard, and that none of the included vehicle models in model years 2006-2008
were equipped with an indirect system. (As mentioned above, new designs of indirect systems
compliant with FMVSS 138 began to appear in model year 2009.)