I have a question regarding the proper air pressure to put in a tire.
Most cars are now carrying tires listing a maximum air pressure of 44 psi on the sidewall. But the car manufacturers usually recommend airing up tires on their vehicles to 30-32 psi.
I called one of our suppliers about what is the proper air pressure to use, and it was explained to me this way:
``We sell a lot of tires to the original equipment manufacturers. What the car manufacturer says is correct.''
However, the OEMs test cars with only two people and a suitcase in them. But often vehicles carry heavier loads, and this can lead to an overloaded situation.
Tire wise and as an added safety element for the driver, it seems to me it would be better to use the higher inflation pressure. The tire would build up less air pressure in hot weather.
I also have another question: Why doesn't the load-carrying capacity of a tire increase incrementally when air is added?
For example, according to the tire pressure and load-carrying capacity tables, a P215/70R16 size tire can carry a maximum load of 1,441 pounds at 26 psi and at 35 psi can carry 1,650 pounds. But when the air pressure is increased to 35-44 pounds the load carrying capacity remains unchanged at 1,650 pounds. Why is this?
I always thought that additional air pressure increases the carrying capacity of a tire/vehicle. I know it does on commercial tires. Can you answer my questions?
Zach's Tire Co. Inc.
Editor's note: We asked Goodyear and Bridgestone/Firestone about Mr. Zacharias' questions and got these responses:
From Goodyear: ``Tires should be inflated to the vehicle manufacturer's recommendations printed on the vehicle door placard or in the owner's manual, not the maximum limit stamped on a tire sidewall.
``Inflating tires to the maximum level stated on the sidewall could lead to accelerated treadwear. The vehicle manufacturer's recommendations state both front and rear inflations for normal and extra loads. Goodyear recommends that motorists follow these recommendations.
``The tires must carry the load of the vehicle, within specifications. Tire load capacity and vehicle load go hand-in-hand, and this is the key to correct tire performance.''
From Bridgestone/Firestone: ``In a general sense, increased pressure does mean increased load capacity for tires. But there are a few caveats.
``One, if the tire is at its maximum pressure for load (P-metric standard load=35 psi; P-metric extra load=41 psi; Eurometric standard load=36 psi; LT-metric load range C=50 psi; LT-metric load range D=65 psi; LT-metric load range E=80 psi), there is no increase in load-carrying capacity with increased pressure. (Some passenger tires are labeled as 44 psi maximum inflation pressure, but the extra 9 psi is for vehicle handling and stability and not for load-carrying capacity.)
``Two, a higher load-carrying capacity for the tire does not necessarily mean a higher load capacity for the vehicle.
``Some vehicles are `tire limited' in that the max load capacity of the vehicle is determined by the tire.
``But other vehicles may be `axle limited' or `suspension limited,' meaning an increase in tire capacity will not help because the limiting factor is elsewhere.''