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.
Proper Tire Pressure: Why it Matters
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, 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. 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. So if you're reading this in December and you last ensured proper tire pressure in August, your tires are now underinflated, guaranteed.
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. See Tires 101 for more information.
Rotate Your Tires on Schedule
Few automotive maintenance terms are as misperceived as tire rotation. The tires rotate every time you drive, right? Right. But here we're talking about the kind of rotation that happens with a volleyball team. All the players move clockwise into the position previously occupied by a teammate. It's the same story with tire rotation, though the pattern is typically more complex than a clockwise shift. We'll address that in a moment. First, the why, how and when:
Why do it? The simple answer — one that dictates both how we buy and how we maintain tires — is that all tires must perform as closely as possible to the others. Tires perform differently when worn than when new: A worn tire (not necessarily a worn-out tire) tends to have better traction on dry roads and respond more quickly to steering changes, because there's less squirm in its shallower tread blocks. However, as a tire wears, its grooves also get shallower, so its ability to shed water and maintain traction on wet roads slowly diminishes. On wet pavement, a newer tire typically prevails.
The way to maximize handling and control is for all four tires to be the same type, to have a consistent degree of wear and to be filled with the correct air pressure. Handling tests show that when any of the above differ — especially between front and rear tire pairs — the car's behavior becomes less predictable, controllable and consistent — even in the hands of a professional driver.
Unfortunately, tires wear at different rates when mounted on the front or rear. Drive wheels tend to wear faster, especially on front-wheel-drive cars where they bear roughly 60 percent of the car's weight and serve to steer as well as propel the car. Rotating the tires allows the wear levels to even out. Diligent rotation means they all wear out, and can be replaced, at roughly the same time. Failure to do so means you're buying two tires at a time and always running with mismatched treadwear.
How to have it done: Most people who bother to rotate their tires will have it done at a service garage, mainly because a technician can put the car on a lift, remove all four wheels (the wheels usually stay with the tires), balance them, rotate them and get you on your way quickly. Firestone Tire Centers, one of the largest nationwide retail tire chains, charges roughly $7 to $10 per rotated tire. Trying to do the same by yourself with a single bumper jack or floor jack makes for an awful lot of jacking and lowering, jacking and lowering, and you won't be equipped to balance the wheels. An unbalanced wheel can produce vibrations and uneven wear to both the tire and the car. Little weights must be added to compensate for natural imbalances in the tire or wheel. Tires can become unbalanced over time, especially if one of the existing weights comes off, and now is the time to correct the situation.
The main mystery is what type of pattern to use when moving wheels from one corner to another. Tires may switch corners, jump into the trunk and kick a full-sized spare out or follow some other pattern. Ask two mechanics or car enthusiasts which of these patterns to use for your car, and you're likely to get three opinions. That's why we recommend that you follow your vehicle manufacturer's directions. If you have this done at a dealership for your brand, they're likely to know what's best for your model.
Things are complicated if you have a full-sized spare that you want to include in the rotation, or if you have directional tires that can only spin in one direction. Most owners will rotate directional tires front to back on the same side, but because directional tires are often high-performance types, purists may choose to have them removed from their wheels and remounted on the opposite side's wheels to expose them to that side's forces. Doing so typically costs more than simple rotation — probably an additional $5 to $20 per remounted tire. If the original tires are larger in back than in front, obviously they won't be rotating forward. Symmetrical tires, which are meant for one side of the car or the other, also never switch sides.
When to do it: This, too, is up for debate. The simple answer is to follow the owner's manual, but their rotation intervals are usually evenly spaced. Some experts say that tires wear more quickly when new than when old. He suggests a more frequent schedule early on, followed by a more relaxed pace as the tires near the end of their life: Rotate every 3,000 miles for the first 15,000 miles, then every 6,000 miles up to 30,000 miles and then every 9,000 miles for the rest of the tire's life, which is about 40,000 miles for the average all-season tire. (Your results may vary.)