One of the most persistent questions we get is, "How much air should I put in my tires?"  Like most things, there is no simple answer, but over the years, we have found guidelines that give very good results.

The first factor to consider is the type of application.  For instance, passenger car tires and load rated truck tires will require different tire inflation practices.  A load rated tire is one that the size designation does NOT start with a P.  For instance a P245/75R16 is considered a P-metric tire and is different from a 245/75R16 Load C tire.  The lack of the P and the addition of a load rating (normally C, D or E for 6, 8 or 10 ply) means this tire is designed to carry a load, as in truck use.

With all tires the maximum inflation, listed on the sidewall, is the maximum the tire should hold.  This number must never be exceeded.  The amount of tire pressure listed on the door placard is normally much lower.  So which is correct?

The answer depends on what is to be achieved.  Engineers determine the amount of air required to safely support the weight of the vehicle.  They also take ride comfort into consideration.  The less air pressure, generally the better the vehicle will ride.  The door placard rating is a minimum and tires should never be inflated to less than this amount.

As tire pressure increases, ride comfort generally decreases.  This is due to the tire flexing less.  At the same time, tire life and fuel mileage may increase with more inflation.  We have found that starting with 10% under the maximum, listed on the sidewall often gives overall better results, with a P-metric tire.  For instance, a tire listed at 44 PSI would start with 40 PSI.  From this point pressure can be adjusted downward if ride quality is too harsh.

With a load rated (not P-metric) tire this may give an extremely harsh ride.  On these tires I normally recommend starting with the door placard and adjusting upward until a good compromise is found.  In either case the minimum nor maximum specified should never be exceeded.

The following pictures may be useful in determining tire inflation:

With proper inflation, wear is even across the entire tread area.  With P-metric tires we find this usually occurs around 10% under the maximum listed on the sidewall.

Load rated tires should start with the amount listed on the door placard and be adjusted upward, based on ride and wear, never to exceed minimum nor maximum.

This is a more common sight, wear from under-inflation.  Both shoulders are worn about the same and far more than the rest of the tire. 

We find this sometimes occurs when  using the inflation pressure listed on the door placard, though ride quality may be enhanced.  Inflation less than listed on the door placard can be dangerous and this minimum should never be exceeded.

Excessive wear to the center of the tread, and little on the shoulders can indicate over-inflation.  Over-inflation can be dangerous and the maximum on the sidewall should never be exceeded.

Tire wear can also result from vehicle misalignment.  For more information on tire wear, please visit our vehicle questions section.