This project was originally published in the May 2002 issue of Popular Mechanics. You can find more great projects at Popular Mechanics DIY Central.
Step 1: Preflight Inspection
These pressure specs usually are lower than the maximum pressure allowed on the tire’s sidewall, but they’re based on each particular vehicle and its rated load, not what the tires are physically capable of withstanding. Although tire pressure specifications usually peak at 32 to 36 psi, buy a tire pressure gauge that reads to at least 60 psi. That’s a typical pressure specification for a compact spare. If the vehicle manufacturer specifies a pressure range, such as a minimum and a maximum, always use the maximum.
The higher the pressure, the greater the load-carrying capacity of the tire, the more stable the vehicle’s handling will be, and the cooler the tire will run at speed. All tires leak air pressure over time. The closer the tires are to the recommended maximum pressure, the greater the safety margin for all operating parameters. A normal tire leaks about .5 to 1 psi per month. In addition, as ambient temperatures drop with the changes of seasons, so do the tires’ air pressure— 1 psi per 10˚F. It might seem that the temperature-related drop is matched by lower operating temperatures of the tires in cold weather.
But the key to tire safety is adequate pressure to carry your vehicle’s load. Although higher tire pressures stiffen the ride somewhat, it’s a small price to pay for the extra safety and the ability of the vehicle to accommodate greater loads. Maintaining proper tire pressure also improves fuel economy, although not by much. Caution: Overinflation increases center tread wear. Don’t forget the spare. And note our earlier warning about the higher specified pressure for a compact spare. If your spare is carried in the underbody, as it is on many minivans, you’ll need to crawl underneath with a pressure gauge and air hose.
Step 2: Inspecting Tires
The side slippage produces friction, which raises tire temperatures and not only increases tread wear but causes the wear to be uneven. So inspect the tire treads. As you do, pry out pebbles from the grooves. They reduce traction and can damage the tread area. The treads should be deepest at midpoint—at least 1⁄8 in. thick. That’s 1⁄16 in. above the tread bars that are the official “replace them” indicators. However, you need tread above those bars or the tire will do a poor job of shedding any water and slush it runs into on the road.
The wear pattern should be relatively even at each side, although it might be somewhat greater in the middle. Tread wear that is “feathered”—worn to a sharp edge at one or both sides— or much greater on one side is a sign of misalignment. If you see cuplike wear in the treads, typically along one side, the possible causes are wheels that are out of balance, worn-out shocks or struts, and loose suspension components. The classic sign of unbalanced wheels is high-speed (50 mph and up) vibration, and it usually surfaces before the cuplike wear becomes noticeable.
Suspension problems usually produce shake at lower speeds. Look for any cuts on the surface of the tire that expose the steel belt or fabric cord. This is grounds for immediate replacement. Tires with unevenly worn treads should be replaced, unless the problem is caught early and there’s plenty of tread depth left. In that case, they could provide a moderate amount of life on the rear wheels, particularly on a front-drive car. However, if you have an all-wheel drive that you push pretty hard, invest in an entire set of new tires.
Step 3: Wheel Balancing
It takes at least a half-dozen differently shaped weights to fit properly on the rims of all the popular wheels. There are several so-called “universal” weights, but they may not fit your rim, and could pop off or cause rim damage. If the wheels are balanced and you have wheel shake at medium speeds on up, or the tires have cup wear, check the shocks and struts.
Step 4: Sidewell Check
Further, professional service is not always expertly done. Unless the lug nuts are tightened to specifications in three even stages, using a crisscross pattern, the rotors may become warped, which adds to maintenance costs. If you let the mileage stretch out a bit, such as to 10,000 miles or more, tires may develop almost imperceptible wear patterns that will affect ride when they’re moved to a new position on the car. If you can’t rotate the tires often, you may be better off leaving them in place and accepting the somewhat shorter tread life. Some tire treads are directional. They should rotate in only one direction and should not be rotated except by a professional who can demount them. How can you tell if you have this type of tire? Look for a directional arrow on the sidewall.
Step 5: Tire Rotation
If you let the mileage stretch out a bit, such as to 10,000 miles or more, tires may develop almost imperceptible wear patterns that will affect ride when they’re moved to a new position on the car. If you can’t rotate the tires often, you may be better off leaving them in place and accepting the somewhat shorter tread life. Some tire treads are directional. They should rotate in only one direction and should not be rotated except by a professional who can demount them. How can you tell if you have this type of tire? Look for a directional arrow on the sidewall.