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Car Care & Repair Tips - June 13, 2003

What Your Mechanic Doesn't Want You to Know


Reader Question: I am not sure what air pressure my tires 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 tire pressure?

Thank you, Andrea

Dear Andrea,

For the answer to your question I turned to the tire experts at the www.Tirerack.com. The information below was found there.

Checking Tire Inflation Pressure

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

The tire usually needs to have additional load capacity as well. This extra capacity is important because without it all of the tire's performance 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 tires 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 tire 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 tire's wear and performance characteristics have also changed.

Under-inflation

If your vehicle's tires are under inflated by only 6 psi it could lead to tire failure. Additionally, the tire's tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tire 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 tires are over inflated by 6 psi, they could be damaged more easily when running over pot holes 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. 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

Tire inflation pressures 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 tires 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 tires 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 tire'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 tires can offer!

Sincerely,

Austin C Davis

Austin C Davis


Interested in an e-book about everything your mechanic doesn't want you to know? Sound advice from Austin Davis. Click Here!


The above article is provided for the interest and entertainment of our visitors. The views expressed in this article are only those of the author, who is solely responsible for the content. AutoGuide.net does not endorse any of these views, and is not to be held responsible for any of the content provided in the above article.


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