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11
Understanding Tire Pressure and Load Limits
Tire inflation pressure is the level of air in the tire that provides it with load-carrying capacity and affects the
overall performance of the vehicle. The tire inflation pressure is a number that indicates the amount of air
pressure– measured in pounds per square inch (psi)–a tire requires to be properly inflated. (You will also find
this number on the vehicle information placard expressed in kilopascals (kPa), which is the metric measure used
internationally.) Manufacturers of passenger vehicles and light trucks determine this number based on the
vehicle’s design load limit, that is, the greatest amount of weight a vehicle can safely carry and the vehicle’s tire
size. The proper tire pressure for your vehicle is referred to as the “recommended cold inflation pressure.” (As you
will read below, it is difficult to obtain the recommended tire pressure if your tires are not cold.) Because tires are
designed to be used on more than one type of vehicle, tire manufacturers list the “maximum permissible inflation
pressure” on the tire sidewall. This number is the greatest amount of air pressure that should ever be put in the
tire under normal driving conditions.
Checking Tire Pressure
It is important to check your vehicle’s tire pressure at least once a month for the following reasons:
•
Most tires may naturally lose air over time.
•
Tires can lose air suddenly if you drive over a pothole or other object or if you strike the curb when
parking.
•
With radial tires, it is usually not possible to determine underinflation by visual inspection.
For convenience, purchase a tire pressure gauge to keep in your vehicle. Gauges can be purchased at tire
dealerships, auto supply stores, and other retail outlets.
The recommended tire inflation pressure that vehicle manufacturers provide reflects the proper psi when a
tire is cold. The term cold does not relate to the outside temperature. Rather, a cold tire is one that has not been
driven on for at least three hours. When you drive, your tires get warmer, causing the air pressure within them to
increase. Therefore, to get an accurate tire pressure reading, you must measure tire pressure when the tires are
cold or compensate for the extra pressure in warm tires.
Steps for Maintaining Proper Tire Pressure
•
Step 1: Locate the recommended tire pressure on the vehicle’s tire information placard, certification
label, or in the owner’s manual.
•
Step 2: Record the tire pressure of all tires.
•
Step 3: If the tire pressure is too high in any of the tires, slowly release air by gently pressing on the
tire valve stem with the edge of your tire gauge until you get to the correct pressure.
•
Step 4: If the tire pressure is too low, note the difference between the measured tire pressure and
the correct tire pressure. These “missing” pounds of pressure are what you will need to add.
•
Step 5: At a service station, add the missing pounds of air pressure to each tire that is underinflated.
•
Step 6: Check all the tires to make sure they have the same air pressure (except in cases in which the
front and rear tires are supposed to have different amounts of pressure).
If you have been driving your vehicle and think that a tire is underinflated, fill it to the recommended cold inflation
pressure indicated on your vehicle’s tire information placard or certification label. While your tire may still be
slightly underinflated due to the extra pounds of pressure in the warm tire, it is safer to drive with air pressure
that is slightly lower than the vehicle manufacturer’s recommended cold inflation pressure than to drive with a
significantly underinflated tire. Since this is a temporary fix, don’t forget to recheck and adjust the tire’s pressure
when you can obtain a cold reading.
Tire Size
To maintain tire safety, purchase new tires that are the same size as the vehicle’s original tires, or another size
recommended by the manufacturer. Look at the tire information placard, the owner’s manual, or the sidewall of
the tire you are replacing to find this information. If you have any doubt about the correct size to choose, consult
with the tire dealer.
