Tire pressure affects stopping distance, fuel economy

Willie Amaya of Lindenhurst is an employee at

Willie Amaya of Lindenhurst is an employee at All Weather Tire as well as a customer. (March 3, 2010) (Credit: Photo by Charles Eckert)

As vehicles have become increasingly trouble-free, motorists have increasingly ignored their tires. Tires have improved markedly over the past few decades in terms of performance and wear, but they still require more attention than most of the car -- much more than they tend to get. The most important steps car owners can take are to keep the tires properly inflated and to rotate them on schedule. The proliferation of tire pressure monitoring systems -- required on all new cars since the 2008 model year -- has been a mixed blessing: They might prevent catastrophic failure, but experts say the simplest types are too liberal and the technology makes some owners think they never have to check their tires again.

Driving on underinflated or overinflated tires compromises any or all of the following.

Stopping distance: Properly inflated tires maximize tread contact with the pavement, and traction along with it. Improperly inflated tires extend stopping distances -- meaning it takes more distance to stop the vehicle in an emergency situation. Unfortunately, the tires may feel and perform no differently under normal driving. By the time the problem is evident, it's too late.

Ride and handling: When the tires have too much air, they're overly rigid, which means they don't absorb as much impact, so the car rides rougher and suspension components wear faster. An overinflated tire's tread may crown, allowing only the center portion to contact the pavement, which decreases traction. When the pressure is too low, the ride might be softer, but the tread may not meet the road uniformly, and steering responsiveness is diminished because of greater flex in the tires' sidewalls.

Fuel economy: Underinflated tires greatly reduce fuel economy. For example, tire expert John Rastetter says a Honda Accord with tires inflated 6 psi below the recommended spec suffers a 5 percent decrease in fuel economy. In an efficient car, this amounts to whole miles per gallon.

Treadwear: Along with the traction decrease that comes with overinflation, it also causes the tread to wear more quickly in the center. Underinflation causes wear closer to the sides and allows more heat buildup, speeding wear. Rastetter says the tires on the Accord example above, inflated 6 psi below spec, would wear out 25 percent faster.

Load bearing: Each tire is rated to carry a maximum amount of weight at a prescribed tire pressure. Some cars specify a higher pressure for greater loads. At best, a tire that's underinflated for the load at hand will suffer the problems detailed above. Extra heat generated in the tire can cause it to fail even if it had held up under a lighter load. So before loading your SUV full of luggage and family members for a road trip, be sure to check its tires.

Tire pressure, including that of the spare tire, must be checked monthly. Tires lose roughly 1 pound per square inch (psi) of air pressure per month, and another 1 psi for every 10-degree Fahrenheit drop in temperature.

Thinking you can check a tire's pressure by looking at it is foolhardy. Some tires look fine when they're underinflated enough to compromise safety, and some tires look low when they're properly inflated. By the time a single tire is so underinflated compared with the others that it's clearly in need of air, it's probably one hard turn, panic stop or trunkload of suitcases away from catastrophic failure.

Unfortunately, tire pressure monitoring systems are designed to alert the driver only if a tire's pressure is 25 percent or more below specification, which is already well into the danger zone. The best-case scenario comes in the form of the most advanced "direct" systems, which combine radio contact using transmitters in each wheel with a display on the dashboard showing the exact pressure -- sometimes including the spare tire. More common in pricier cars, this display saves you the trouble of taking a gauge to dirty wheels. The driver does, however, still need to know what the proper pressure is and must bother to check the display regularly.

Find your tire pressure specs

Before you can maintain the proper tire pressure, you must know for certain what that pressure should be, in psi. The proper tire pressure rating is not labeled on the tire itself -- a common misconception. That number is the maximum rating for the tire irrespective of the vehicle on which it's used. Invariably, it's dramatically higher than the pressure you should be using. If you own a brand-new car, this task will be easier, as the federal government now requires more conspicuous and clear tire information labels in a standard place: on the driver's door frame. If your car is older, it could be in the owner's manual, on the fuel-filler door, on any of the other doors or doorjambs, in the glove compartment, in the center storage console or even in the engine compartment.

All vehicles show at least three separate pressure ratings: for the front tires, the rear tires and the spare tire (unless the car employs run-flat tires to do away with a spare, or a deflated ultra-compact spare that comes with a compressor to inflate when needed). Sometimes these placards can be confusing: Especially among European imports in years past, the tire information label has shown the specs for every possible tire size and type the automaker offered when the car was new. Sometimes they include separate specs for when the vehicle is lightly loaded and when it's full of occupants and cargo -- a difference as great as 10 psi for some cars. If you find this high volume of data on your car's tire-spec placard, make sure to look for the size marked on your tire's sidewall to know which spec is the right one for your car.

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