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