The tires, taken together, probably comprise the single most underrated component on your automobile--underrated even by many experts. Most people regard them as sort of..well, there...just good foot soldiers doing the bidding of the car to which they are attached. In fact, the tires on your car play a crucial role in determining a whole host of performance factors ranging from safety and handling to economy and riding comfort. Top automobile industry engineers know that a brilliant design can be seriously compromised by poorly matched or inadequate tires. That's why the auto companies work so closely with the tire manufacturers to develop and offer a wide variety of tires on their new cars, with each model having the specific size and design most appropriate to it. If auto engineers take the subject that seriously, you should, too.

Tire Specifications
The tires that came with your car as standard equipment when the car was new were carefully selected to afford a good balance of the following qualities:

• Durability
• Handling
• Traction
• Riding comfort
• Low noise level
• Low rolling resistance
• Tread mileage
  

On most late-model cars the precise specifications of your tires appear on a so-called "tire placard" located on the jamb of the driver's door. This plate gives tire sizes as well as cold inflation pressures (after the car has been sitting for several hours) and maximum vehicle load recommendations for normal driving conditions. These are the design limits of not only the tires, but of the car itself, so it is unwise ever to load your car beyond these load recommendations.

Tire Inflation Pressure
Inflation pressure is the single most important factor in getting the most out of your tires. Under-inflation can cut tire life, reduce safety and handling and take a severe toll in fuel economy. This is because under-inflation increases tire flexing, raises rolling resistance and causes excessive heat build-up. The only performance quality that improves with under-inflation is riding comfort--having too little air pressure will cause a tire to ride "softer"--but at a high price in terms of all the other desirable tire qualities listed above. Inflation pressures that are too high, on the other hand, can improve handling and rolling resistance, but result in abnormal wear patterns, a harsh ride and can increase the chances of sustaining serious damage from road hazards. (If you want performance qualities different from those for which your tires were designed, buy a new set of tires that are designed to reflect your needs. "Quick fix" solutions such as tampering with recommended inflation pressures are not a realistic answer over the long haul.)

Inflation pressures should be checked at least once a month while the tires are still "cold"--in other words, after they have been sitting for at least three or four hours and driven less than a mile. This will be the accurate pressure reading. Tire pressures will increase as a car is driven by as much as 4-8 lbs. depending on the type of driving. Outside air temperature also has a similar effect on tire pressure. It doesn't require as much air to achieve a given pressure reading in August as it does in February. That is another reason why tire presure should be checked on a monthly basis.

When checking inflation pressure always use a tire gauge made for the purpose. These are inexpensive and are available from any good auto parts store. Simply looking at the tires is not a good indication of inflation level. The sight test was easier to do by experienced drivers when bias-ply tires were common, but modern radials look "flat" even when properly inflated.

If you experience a flat or a sudden loss of air pressure while driving, stop the car as soon as possible and summon help. Flat tires can often be repaired inexpensively, but driving on them while flat or seriously under-inflated can cause severe damage that will drastically reduce tire life or even render them irreparable. Certain tires, like Michelin's Run Flat Tire, are designed to assist you at a time when you've got a damaged or desparately low tirel. Make sure, though, that your tires have this feature before continuing to drive. If this is you, these tires are not meant for normal driving, but just to get you, immediately, to your local mechanic for a replacement.

Tire Wear Inspection
Tires should be inspected for wear at least once a month. Look for abnormal or uneven wear patterns and for cuts or bulges. Uneven or abnormal wear patterns can be caused by improper inflation, improper balance, bad wheel alignment, poor driving habits and lack of regular rotation. Any problems observed should be checked by an expert.

Tire Rotation
The front and rear tires on your car do different jobs and are subjected to different stresses. For this reason, it is essential for maximum tire life that they be rotated at regular intervals. Many tire manufacturers recommend rotation every 7,500 miles, but follow the specific guidelines for your brand and type of tire. This information should be part of the tire warranty material that came with your tires when new, or it can be obtained by visiting or calling a tire store handling your brand.

Bias and bias-ply tires, generally found on older automobiles, are rotated using the optional "X" rotation pattern. Radial tires, on the other hand, are generally not rotated from from left to right, but simply from front to back, also as indicated in the diagram.

Wheel Alignment and Tire Balance
Proper wheel alignment will improve the handling of your car and the life of your tires. Alignment should be checked whenever your tires are rotated, or sooner if you any steering problems, such as "pulling" to the left or right while driving.

Proper tire balance will improve riding comfort and increase tire life. Improper balance will cause uneven wear such as cupping and flat-spotting and noticeable, occasionally dangerous, vibration.

When to Replace Tires
You should replace any tire that shows any of the following defects:

• Dangerous wear.
• Irreparable punctures.
• Other serious damage.
  

If the cord or fabric molded inside the tire is showing or if the tread is worn to a point where less than two-thirty-seconds (2/32) of an inch remains, that tire should be replaced. Most tires have built-in tread wear indicator bars that appear between the tread grooves when the tire is worn below a safe level (see diagram). When these indicator bars are visible at more than one spot around the tire, the tire should be replaced.

If a tire is punctured that is not always an indicator of whether it needs to be replaced. sometimes tire punctures can be "plugged." And, recently, there are some new spray bonding that you can use to plug it up yourself. I suggest that if you're not the car repair typeof person, leave it to the professionals. Besides, it's rather inexpensive.

Cracks, splits or cuts in the tread or sidewall deep enough to expose the inner cord or fabric of the tire should be check by a professional mechanic and possibly replaced. Likewise, Bumps and bulges anywhere on a tire indicate replacement.

Buying Replacement Tires
You should always be careful to follow the manufacturer's specifications when replacing original equipment tires. Never buy less tire than the car had new. While it is possible to upgrade your tires from original equipment specifications, care should be exercised there, too. A high performance tire that really hugs the road and produces great handling on dry pavement may cut your gas mileage (because of increased rolling resistance) and be very tricky to handle on wet surfaces.

In most cases, the precise specifications of your current tires are molded right into the sidewall (see diagram). If you choose to buy a replacement tire or tires that differ in specification, be sure you select ones that have the same size, load range and construction type (bias, bias-belted or radial). A different size or type of tire can adversely affect not only ride and handling, but such disparate factors as speedometer calibration and ground clearance.

WARNING TO OLDER CAR BUYERS: For reasons of safety, you should never mix tires of different construction types on the same car. Bias, bias-belted and radial tires are not compatible. If you wish to switch tire type, and have made sure that your car will handle the new type, switch all four (or five if you have a full-size spare) at the same time.

If you replace only a single tire, make sure it is fully compatible with those already on your car. Then, place the new tire on the same axle with the least worn tire of the other three.

Summary.
Don't take your tires for granted. They are among the most critical components of your car, with an important bearing on the safety and enjoyment you derive from your vehicle.