The issue of underinflated tires and their role in highway accidents has received lots of press during the past year. Congress weighed in by passing the TREAD Act, which mandates that NHTSA address the issue directly. In response, NHTSA recently issued an Advanced Notice of Proposed Rulemaking, “Tire Pressure Monitoring Systems.” (See www.nhtsa.com, Docket No. 8572.) My concern is that while born of good intentions, this response may be an oversimplified solution to a complex issue.
Many interpret the TREAD Act as requiring that all new highway vehicles include a warning indicator to alert drivers of “substantially underinflated” tires. If used appropriately, it could be a good thing. Slow air loss caused by punctures, leaking valves and even lack of inflation checks could be detected based on some minimum threshold pressure reference. But it's not that simple.
Many issues have yet to be decided. Will individual wheel positions have to be identified separately on the dashboard display? Will displays be standardized for all types of vehicles? Will the requirements apply to vehicles that already have more sophisticated and expensive onboard inflation control systems? How will adjustments be made for cold vs. hot inflation pressures and big differences in ambient temperatures?
And who defines minimum acceptable pressure? Should it be the tire or vehicle application engineer? Should it be based strictly on load carrying capacity of the tire, or should it also include vehicle-handling variables at different inflation pressures? Since unbalanced pressures can affect vehicle stability, should overinflated tires be flagged as well? Are inflation-pressure warning level adjustments in order when the vehicle is towing a trailer or is carrying excessively heavy loads?
Also, there are tires with the same basic size designation, but a different prefix or suffix to indicate passenger or light-truck versions. Nearly all pickups and SUVs come with passenger tires. However, some users choose the more robust light-truck version when they replace the tires. This means that tires with the same size designation could have very different load/inflation relationships.
The Tire and Rim Assn. has a long-established standard that should be used when selecting passenger tires for use on light trucks, trailers and multi-purpose passenger vehicles. A 1.10 service factor compensates for increased side loadings and higher centers of gravity. For example, if a passenger tire must actually carry 1,200 lb. when used on a pickup, it must be selected and inflated so that it's rated to carry 1,320 lb. When spec'ing heavy-duty highway trucks, factors such as ground clearance, platform heights and gear ratios must also be taken into consideration. Thus, it's possible to spec tires with different load/inflation relationships for identical loads.
Since determining the correct tire pressure for a particular vehicle is not a simple task, the kind of monitoring devices proposed by the federal government may not be the answer to the problem of underinflation. There's also a lot we don't know about the tire failures that triggered Congress' original investigation into this issue. We don't know how many were caused by underinflated tires as opposed to long-term lack of maintenance or recent damage. And we don't know the role played by driving skills. Did drivers take vehicle size and type, as well as road conditions, into account when they made adjustments in speed, cornering and braking distance?
An even greater concern is that a dashboard warning device may give drivers a false sense of security, leading them to eliminate tire checks or tire-related maintenance. That would be the ultimate disservice.