Apathetic vehicle owners usually get blamed for failing to maintain adequate air pressure, but inaccurate gas station air gauges that overstate pressure are causing even some conscientious motorists to unknowingly underinflate their tires, government studies show.
Nationwide surveys funded by the National Highway Traffic Safety Administration (NHTSA) showed many gauges used in gas stations tended to overstate tire pressure-thereby leading motorists to underinflate their tires.
A number of hand-held gauges tested also tended to overstate inflation pressure but to a lesser extent than those typically found at gas station air outlets. For that reason, NHTSA recommended that motorists purchase and use a hand-held gauge when checking their tire pressure.
Research was conducted in 2001 by the National Center for Statistics and Analysis, a unit of NHTSA, in compliance with the Transportation Recall Enhancement, Accountability and Documentation (TREAD) Act.
In announcing the findings, NHTSA pointed out that gas station air outlets play a significant role in highway safety and energy conservation, since ``the typical motorist owns neither an air pump nor a pressure measuring gauge.''
Studies were intended to measure such factors as the typical motorist's chances of finding an air pump at a gas station, the likelihood the pump is working, whether a fee is charged for the pump's use, the probability that an air pressure gauge is present and, if so, how accurately the gauge performs its task.
Researchers found it still is commonplace for gas stations to make air pumps available to the public. Based on survey data, researchers estimated air pumps can be found in 94 percent of the gas stations in the U.S. However, data also showed that nearly one in 10 air outlets (9 percent) is likely to be out of service.
Nationwide, fewer than half the gas station air outlets surveyed (49 percent) were equipped with a pressure gauge, researchers found.
Survey results suggested an estimated 43 percent of U.S. gas stations charge a fee for pump use-a practice more common in the South than in other regions. There, about 63 percent of those stations surveyed charged a pump fee.
The fact that a station charges a pump fee nearly doubles chances that a pressure gauge also will be provided, researchers found. Ironically, the existence of such a fee had the opposite effect on the probability that the station's air pump will be working (lowering it from 95 to 86 percent).
NHTSA said its research also suggested that the higher the recommended pressure, the greater the probability that gas station gauges will overstate actual tire pressure.
Within the 25- to 35-psi pressure range-that recommended for most passenger cars and sport-utility vehicles-nearly one gas station gauge in five was found to overstate tire pressure by at least 4 psi.
At 35 psi, nearly one in 10 station gauges was found to give a reading that exceeded actual tire pressure by at least 6 psi. Thus, motorists who think they're inflating to 35 psi will drive away having an actual tire pressure of 29 psi or less, NHTSA said.
In conducting the study, researchers used a seven-gallon Air Works Portable Air Tank, pressuring it to four reference levels (25, 35, 45 and 55 psi) using the station's air pump. Pressure readings of the station's air gauge (if present) then were compared with those of the researcher's gauge (a Longacre Model No. 50402-0-60).
Seventeen hand-held gauges were checked for accuracy. They also tended to overstate rather than understate inflation pressures. Unlike the station units, however, the hand-held units were less likely to overreport tire pressure by 4 psi or more. And unlike station gauges, none of the hand-helds overreported by more than 8 psi. At 55 psi, nearly one gas station gauge in 10 overstated tire pressure by this amount, NHTSA said.
Digital hand-held units, although more expensive than so-called pencil-type gauges, performed best of all in the tests, showing a mean deviation in pressure of 0.1 psi or less.