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Do you Understand your Tires?

Want to read your tire's thoughts and intentions? You should, because understanding what your car's shoes can and cannot do is as important to driving, as is the car's horsepower rating or rear suspension design. As with any component of the car, if you learn the limitations of the tires, and stay within them, you will benefit from all of the performance, safety, and reliability advantages they have to offer. It is not as tough as one might think either because the manufacturer has conveniently provided you with all of the information you need right on the sidewall. It is very cryptic though, so you have to be able to speak the lingo.

This particular month, I happen to be smitten with Acura's 3.2CL Type S, which is shod with P215/50R17 93V Michelin MXM4 tires; so let us use that as an example. P215 indicates the width of the tire in millimeters. 50 is the aspect ratio, or the height of the sidewall. R indicates that the tire is radial construction, and 17 indicates diameter of the wheel in inches, on which the tire fits.

The next number is the load index, which indicates the maximum amount of weight the tire can handle. The CL's tires are rated at 93, meaning that each one can handle 1,433 pounds. If you multiply that by four tires, you get a total weight of 5732. Then subtracting the CL's weight of 3,446 pounds (Type S, six-speed), you find that the car can carry 2,286 pounds of people and cargo. This figure is high but then again, the example is that of a good tire.

Important, basic tire information, (inflation pressure and weight limitations) are usually also printed near the driver somewhere, such as inside the driver side doorjamb. This will give you exact poundage that the tires can handle, passengers and cargo, which may save you some math. Tires can generally hold between 853 (75) and 2679 (115) total pounds. Be very careful though with SUV's. Some, especially car-based SUV's, look tough enough and have plenty of interior space. However, they actually cannot haul much more than cars, and this gets their owners into trouble on the roads. Some common load index examples would be;

  • 79 = 963 pounds
  • 86 = 1,168 pounds
  • 93 = 1,433 pounds
  • 100 = 1,764 pounds
  • 107 = 2149 pounds

The final letter is, in my mind, the most interesting one; the speed index. All American tires meet the government's minimum capability of traveling at 85 miles per hour without heat or road conditions causing breakdown of the tire construction. Some cars though require a higher rating because their top speed capabilities are higher. Speed index works about like this;

  • P = 94 mph/150 kph
  • Q = 100 mph/160 kph
  • R = 106 mph/170 kph
  • S = 112 mph/180 kph
  • T = 118 mph/190 kph
  • U = 124 mph/200 kph
  • H = 130 mph/210 kph
  • V = 149 mph/240 kph
  • Z = 149+ mph/240+ kph

You may also notice three other pieces of information on your sidewall; the tread wear, traction, and heat resistance ratings. Tread wear should be given in the form of a three digit number, such as 300 or 320. To estimate your tires expected life under what the manufacturer considers normal conditions (check your owner's manual for these conditions), multiply the number by 200 and you will get your mileage.

For example, a rating of 360 should allow the tire to see 72,000 miles. The traction rating is measured as an A, B, or C, and describes the tires' abilities to grip on wet surfaces (based on design and tire composition). Unless you live in the Sahara and want to save a little money, you will want the best rating, which is A. Heat resistance is also measured with an A, B, or C. This indicates how well the tire resists friction heat build up during high speeds. A, resists better than B, which resists better than C. Again, you will want A as it is more resistant to blow-outs and tread separation at high speeds. In addition, because the tire runs cooler, it stays firmer, causing less resistance and making it marginally more fuel-efficient.

In most cases, I have found that the automobile manufacturer does a good job of pairing the proper tire to a car. The speed rating of the tire chosen will naturally align with the car's performance capabilities. Tread, heat, and traction ratings on new car tires, are generally all more than acceptable these days. Information such as this is more helpful when replacing the manufacturers original units down the line. If you are satisfied with your tires performance, simply replace it with the same product that the car manufacturer specified when new.

Be warned though, that this can cause chronic sticker shock. There is another option, though. It is also perfectly acceptable to ask your tire dealer for a replacement tire that fits and has the same performance ratings as your old tire, but costs less money. A reputable tire dealer should be able to provide you with satisfactory alternatives that can save you a bundle. All you have to do is ask.