HOW DO YOU KNOW WHAT PRESSURE YOUR CAR SHOULD HAVE??

I am not sure what air pressure my tyres should have in them. I do not see anything marked on the tires themselves, and I do not have my owners manual. How can I find out the proper tyre pressure?
Thank you,
Andrea

Checking Tyre Inflation Pressure

When vehicle manufacturers select a tyre size for a vehicle, they evaluate the vehicle's gross axle weights, the anticipated use of the tyre, and the tyre diameter and width. Adjustments to these factors give the manufacturer a way to improve handling and appearance. This is especially true for performance tyre sizes. The size selected is rarely limited to only one capability (i.e. carrying the vehicle's weight).

The tyre usually needs to have additional load capacity as well. This extra capacity is important because without it all tyre pressure would be used up just carrying the weight of the vehicle and little would be left for durability at high speeds or responsive handling. For all vehicles produced since 1968, the original tyres sizes and inflation pressures (including the spare) are listed on a vehicle placard. This placard can be located on:

• The driver-side door or door jamb (Ford vehicles on the rear passenger door jamb)
• Glove box or counsel door
• Fuel filler door
• The engine compartment

Additionally, some manufacturers also list the original tyre pressure in the vehicle's owner's manual. If a car's inflation pressure has varied from that which was recommended by the manufacturer, it's likely that the tyre's wear and performance characteristics have also changed.

Under-inflation

If your vehicle's tyres are under inflated by only 6 psi it could lead to tyre failure. Additionally, the tyre's tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tyre to deflect (bend) more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. You would find a significant loss of steering precision and cornering stability. While 6 psi doesn't seem excessively low, remember, it usually represents about 20% of the tire's recommended pressure.

Over-inflation

If your tyres are over inflated by 6 psi, they could be damaged more easily when running over pot holes or debris in the road. Higher inflated tyres 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. This is why participants who use street tires in autocrosses, track events and road races, run higher than normal inflation pressures.

Effects of Time and Temperature

change due to time and temperature. Tires lose about 1 psi per month due to air escaping through the rubber as it stretches. If you were to check your tyres only every six months, it would not be uncommon to find them under inflated by as much as 6 psi. Tire inflation pressures also fluctuate with changes in the outside air temperature. This occurs at a rate of about 1 psi for every 10°F (plus or minus). So the tyres you set correctly with an 80°F ambient temperature will be under inflated by 6 psi at 20°F.

If you add the variations of time and temperature together, it is easy to understand why a tyre's inflation pressure should be checked frequently. Improper inflation can cause tires to wear irregularly and can void manufacturer's warranty.

Another advantage of checking tire pressure frequently is that it allows a slow leak to be found and repaired before it permanently damages a tire. Tire pressures should be checked once a week, preferably before the vehicle has been driven. Spending about two minutes a week will help you get the optimum performance your tyres can offer!