Tire inflation is the most ignored item of auto maintenance. Gasoline mileage can drop as much as 0.8% for every 1 pound per square inch (psi) of under inflation.
Two items should be permanent fixture in every glove compartment; a tire pressure gauge and a tread depth gauge. Check the tire air pressure (including the spare) regularly with a pocket type gauge. Kicking the tires won't tell you a thing, and gauges on the service station air hoses are notoriously inaccurate.
The tire pressures recommended for your car are usually found on a tire placard, located on the driver's door or the glove compartment door. Ideally, inflation pressure should be checked when the tires are cool. When the air becomes heated it expands and the pressure increases. Every 10° rise (or drop) in temperature means a difference of 1 psi. (6.895 kpa), which also explains why the tire appears to lose air on a very cold night. When it is impossible to check the tires cold, allow for pressure build-up due to heat. If the hot pressure exceeds the cold pressure by more than 15 psi. (103.425 kPa), reduce your speed, load or both. Otherwise internal heat is created in the tire. When the heat approaches the temperature at which the tire was cured, during manufacture, the tread can separate from the body.
Before starting a long trip with lots of luggage, you can add about 2-4 psi to the tires to make them run cooler, but never exceed the maximum inflation pressure on the side of the tire.
See Figures 1, 2 and 3
All tires made since 1968 have 8 built-in tread wear indicator bars that show up as 1 / 2 in. (13mm) smooth bands across the tire when 1 / 16 in. (1.6mm) of tread remains. The appearance of 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.(1.6mm) tread.
You can check your worn tread depth with an inexpensive gauge or by using a Lincoln head penny. Slip the Lincoln penny into several tread grooves. If you can see the top of Lincoln's head in 2 adjacent grooves, the tires have less than 1 / 16 in. (1.6mm) 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, its's time to replace the snow tires.
See Figure 4
Tire wear can be equalized by switching the position of the tires about every 6000 miles, (9654 km). 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 vehicle to the other); because of their wear patterns and because they would last longer if their direction of rotation is not changed. The manufacturer's tire rotation recommendations for most late model vehicles covered by this information now allows for, and even suggests, cross-switching radial tires to allow for more uniform tire wear.
Some specialty tires may be directional (certain snow or performance tires), meaning they may only be mounted to rotate in one direction. Some 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.
If you have any doubt as to the correct rotation pattern for the tires which are currently mounted on your vehicle, consult the tire manufacturer or one of their facilities for recommendations
When radials or studded snows are taken off the car, mark them, so you can maintain the same direction of rotation.
Store the tires at proper inflation pressures if they are mounted on wheels. All tires should be kept in a cool, dry place. If they are stored in the garage or basement, do not let them stand on a concrete floor; set them on strips of wood.