See Figure 1
Tire inflation is probably the most ignored area of auto maintenance. Gasoline mileage can drop as much as 0.8% for every 1 pound per square inch (psi) of under inflation. Proper tire inflation is also a very important factor in the handling and safety of the car. Tire lift is also affected by air pressure.
Tires should be checked weekly for proper air pressure. A chart, located either in the glove compartment or on the driver's/passenger's door, gives the recommended inflation pressures for your car depending on type of tires used (radial or bias-ply) and whether the car is loaded or unloaded.
Tire pressures should be checked before driving when the tires are still cool. Approximately every 10°F change in tire temperature means a difference of 1 psi, which explains why your car's tires look low on a cold morning. Two items should be a permanent fixture in every car's glove compartment: a tire pressure gauge and a tread depth gauge. Never trust the gauge that is built into service station air pumps as they are notoriously inaccurate.
Before starting on a long trip with lots of luggage, add about 2-4 psi to the tires to make them run cooler, but never exceed the maximum inflation pressure marked on the side of the tire.
See Figures 2, 3 and 4
All tires made since 1968 have 8 built-in tread wear indicator bars that show up as 1 / 2 in. (13mm) wide smooth bands across the tire when 1 / 16 in. (1.5mm) of tread remains. The appearance of these tread wear indicators means that the tires should be replaced. In fact, many states have laws prohibiting the use of tires with less than 1 / 16 in. (1.5mm) tread depth. Unusual wear may indicate front end alignment problems.
You can check your own tread depth with an inexpensive gauge or by using a Lincoln head penny. Slip the penny into several tread grooves. If you can see the top of Honest Abe's head in 2 adjacent grooves, the tires have less than 1 / 16 tread left and should be replaced. You can measure snow tires in the same manner by using the tails side of the Lincoln penny. If you can see the top of the Lincoln Memorial, it's time to replace the snow tires.
See Figure 5
Tire wear can be equalized by switching the position of the tires about every 5,000-6,000 miles (see the accompanying diagram). The pattern you use depends on whether or not your car has a full-sized usable spare or a undersized "donut" spare. Because the compact or donut spare tire is designed for limited emergency use, it should not be included in normal tire rotation. Including a conventional spare in the rotation pattern can give up to 20% more tire life.
Due to their design, radial tires tend to wear faster in the shoulder area, particularly in the front positions. Radial tires in non-drive locations, may develop an irregular wear pattern that can generate tire noise. It was originally thought the radial tires should not be cross-switched (from one side of the car to the other); because of their wear patterns and that they would last longer if their direction of rotation is not changed. The manufacturer's recommendations for tire rotation allows and even suggest cross-switching radial tires to allow for more uniform tire wear.
Some specialty aftermarket tires may be directional, meaning they may only be mounted to rotate in one direction. Some snow tires and special performance tires/wheels will fall into this category and will be marked with directional rotation arrows on the tire sidewalls. NEVER switch the direction of rotation on tires so marked or poor performance/tire damage could occur. This should be taken into consideration in choosing a rotation pattern for directional tires.
Store tires at proper inflation pressures if they are mounted on wheels. Mark radial and studded snow tires with an arrow showing direction of rotation so they can be mounted the same way. All tires should be kept in a cool, dry place. If tires are stored in the garage or basement, do not let them stand on a concrete floor; lay them down on strips of wood.
BUYING NEW TIRES
When buying new tires, give some thought to the following points, especially if you are considering a switch to larger tires or a different profile series:
- All four tires must be of the same construction type. This rule cannot be violated. Radial, bias, and bias-belted tires must not be mixed.
- The wheels should be the correct width for the tire. Tire dealers have charts comparing tire and rim compatibility. A mismatch will cause sloppy handling and rapid tire wear. The tread width should match the rim width (inside bead to inside bead) within an inch. For radial tires, the rim width should be 80% or less of the tire (not tread) width.
- The height (mounted diameter) of the new tires can change speedometer accuracy, engine speed at a given road speed, fuel mileage, acceleration, and ground clearance. Tire manufacturers furnish full measurement specifications.
- The spare tire should be usable, at least for short distance and low speed operation, with the new tires.
- There shouldn't be any body interference when loaded, on bumps, or in turns.