In addition, a tire must transmit handling (acceleration, braking, cornering) to the road.  Cornering forces are transmitted to the rim in a similar manner to load.  Acceleration and braking forces rely on the friction between the rim and the bead.  Inflation pressure also supplies the clamping force, which creates friction.

A tire also acts as a spring between the rim and the road.  This spring characteristic is very important to the vehicle's ride.  Too high an inflation pressure causes the tire to transmit shock loads to the suspension and reduces a tire's ability to withstand road impacts.  Too low an inflation pressure reduces a tire's ability to support the vehicle's load and transmit cornering, braking, and acceleration forces.

Temperature Effects: Air pressure is affected by temperature.  The air under pressure in a tire is no exception.  Typically, an inflation pressure can change by 1 psi for every 10 degrees Fahrenheit of temperature change.  Higher temperature means increased pressure.

Time Effects: Air pressure is also affected by time even if the vehicle is not used.  A loss of 1 psi per month is considered normal.
I.E... If a tire is inflated to 35 psi on an 80° July day, it could have an inflation pressure of 23 psi on a 20° day six months later in January.  This represents a normal loss of 6 psi over the six months and an additional loss of 6 psi due to the 60° temperature change.  At 23 psi, this tire is severely under-inflated.

Proper inflation is the single most important factor in tire care.  The tire sidewall displays MAXIMUM pressure - based on MAXIMUM load capacity of the tire.  The inflation pressure displayed on the door jamb is the RECOMMENDED pressure - based on normal load of a passenger car and MAXIMUM load capacity of a pick-up truck.  While a single pressure setting may be okay for a passenger car, pick-up trucks by nature (especially 3/4- and 1-ton) vary greatly in weight.  The recommended pressure setting for a loaded truck will result in a grossly over inflated tire when the truck is not loaded.  Always check inflation when tires are COLD, that is when the vehicle has been driven less than a mile, or at least one hour or more after driving.  Use a good quality tire pressure gauge.  Note: It is natural for radial tires to have a slight bulge in the sidewall at their proper inflation pressure.

IMPORTANT...
With an "Air" fill check inflation pressure at least once a month. With a "Nitrogen" fill the intervals can be 3 to 6 months depending on age and condition of tire.  In either case always check before any long trip or anticipated travel with a heavy load.  Finally, be sure to check the spare tire pressure at least every 3 months (possibly at oil change intervals).

Proper Tire Inflation

UNDER INFLATION
Under inflation is the most common cause of failures in any kind of tire and may result in severe cracking, component separation or "blowout," with unexpected loss of vehicle control and accident.  Under inflation increases sidewall flexing and rolling resistance resulting in heat and mechanical damage.  Under inflation can cause can cause many tire-related problems.  Since a tire's load capacity is largely determined by its inflation pressure, under inflation results in an overloaded tire.  An under inflated tire operates at high deflection resulting in decreased fuel economy, sluggish handling and may result in excessive mechanical flexing and heat buildup leading to catastrophic tire failure.  Additionally, the tire's tread life could be reduced by as much as 25%.  Lower inflation pressure will allow the tire to flex more as it rolls.  This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%.  This also results in a significant loss of steering precision and cornering ability.

Wear On Both Edges: Cause... "UNDER INFLATION"

If your tire looks like this, it may be under inflated.  Not having enough air in a tire is singly the worst thing you can do to a tire.  Under inflation reduces tread life through increased tread wear on the outside edges (or shoulders) of the tire.  It also generates excessive heat which reduces tire durability and can lead to tire failure.  Finally, it reduces fuel economy through increased rolling resistance (soft tires make your vehicle work harder).

OVER INFLATION
If your tires are over inflated, they could be damaged more easily when running over potholes or debris in the road.  Higher inflated tires cannot isolate road irregularities well, causing them to ride harsher.  However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point.

Tire footprint and traction are reduced when van, pickup, or RV tires are over inflated for the loads carried.  In particular, tires with aggressive tread patterns may contribute to over steer or "road-walk" if inflated beyond the inflation pressure specified in the owner's manual and vehicle placard for standard or customary loads.  Over inflation also increases the chances of bruise damage.

Wear In Center: Cause... "OVER INFLATION"

When a tire has too much air in it, the center of the tread bears most of the load and wears out faster than the outside edges.  If a tire wears unevenly, the useful life is reduced and conversely the operating cost is increased.  Additionally stopping distances will be increased because less tread area is contacting the road surface.

CHECKING TIRE FOOTPRINT WITH A CHALK LINE

	Park on level ground and aim front tires straight ahead. Draw a line completely across the tread. Pull ahead 2-3 tire revolutions or until the chalk starts wearing off. If the chalk mark is worn off equally across the width of the tread, the inflation pressure is correct.
	If the chalk mark is worn off more in the center than at the edges, as shown here, the tire is over inflated.
	If the chalk mark is worn off more at the edges than in the center, as shown here, the tire is under inflated.