Tire inflation is the most ignored item of auto maintenance. Gasoline mileage can drop as much as 0.8% for every 1 psi of under inflation.
Two items should be a permanent fixture in every glove compartment: a tire pressure gauge and a tread depth gauge. Check the tire pressure (including the spare) regularly with a pocket type gauge. Kicking the tires won't tell you a thing, and the gauge on the service station air hose is notoriously inaccurate.
A plate located on the door or in glove box will tell the proper pressure for the tires. Ideally, inflation pressure should be checked when the tires are cool. When the air becomes heated it expands and the pressure increases. Every 10°F rise (or drop) in temperature means a difference of 1 psi, 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, reduce vehicle 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.
For maximum satisfaction, tires should be used in sets of five. Mixing of different types (radial, bias-belted, fiberglass belted) should be avoided.
Conventional bias tires are constructed so that the cords run bead-to-bead at an angle. This type of construction gives rigidity to both tread and sidewall.
Bias-belted tires are similar in construction to conventional bias ply tires. Belts run at an angle and also at a 90° angle to the bead, as in the radial tire. Tread life is improved considerably over the conventional bias tire. The radial tire differs in construction but instead of the carcass plies running at an angle of 90° to each other, they run at an angle of 90° to the bead. This gives the tread a great deal of flexibility and accounts for the characteristic bulge associated with radial tires.
If radial tires are used, tire sizes and wheel diameters should be selected to maintain ground clearance and tire load capacity equivalent to the minimum specified tire. Radial tires should always be used in sets of five but in an emergency, radial tires can be used with caution on the rear axle only. If this is done, both tires on the rear should be of radial design.
Snow tires should not be operated at sustained speeds over 70 mph.
On four wheel drive vehicles, all tires must be of the same size, type and tread pattern, to provide even traction on loose surfaces, to prevent driveline bind when conventional four wheel drive is used and to prevent excessive wear on the center differential with full time four wheel drive.
See Figures 1, 2 and 3
All tires made since 1968, have 8 built-in tread wear indicator bars that show up as 1 /