The useful life of a tire is only five to seven years. For cars and trucks driven every day, the tread usually wears out in less than five years. For RVs that sit for a good part of the year, five years can pass with a lot of tread still left on the tire.
Although you may not want to replace what looks like a perfectly good tire, riding on tires more than five years old greatly increases the risk of a blowout.
Date codes: Every tire has a date code stamped on the sidewall, which gives the date that the tire was manufactured. They look something like this: DOT PDHH MLOR 3403. The date code can be on either side of the tire, so you may have to crawl underneath the rig and look on the inward-facing side. The date code always starts with the letters DOT and ends with a 3- or 4-digit number. That last number is the date code, which tells you when the tire was manufactured. The first two numbers indicate the week (out of 52), and the last one or two digits indicate the year. For instance, 3403 means the 34th week of 2003, or the last week in August 2003. Starting with the year 2000, the date codes have two digits for the year, before that, only one. A date code of 079 would indicate the seventh week of 1999, or the third week of February 1999.
Tires deteriorate with age, even when sitting on a shelf, so always ask to see the date code when you purchase new tires and insist on tires manufactured within the last few months. The tire dealer may give you a funny look because most consumers don’t know about date codes.
Tire size designations: That jumble of letters and numbers on the sidewall of the tire is the tire size designation. The first letters indicate the type of tire: P for passenger car, LT for light truck, and ST for a special trailer. Bus and medium-duty truck tires have no such designation. The next number is the width of the tire, given in millimeters, followed by a slash. The number following the slash is the ratio of width to section height (only relevant to tire engineers) followed by a letter: R for radial ply or D for diagonal or bias ply. It ends with a number which gives the inside diameter of the tire in inches. A tire with the designation ST225/75R15 is a special trailer tire that is 225 millimeters wide with a width to section height ratio of 75. It is a radial ply tire that will be mounted on a 15-inch wheel.
Load range: The load range of a tire is indicated by a letter, A through E, and is stamped on the sidewall of the tire. Tire charts, available from any tire dealer, have these letters in parentheses after some of the tire load limits. The letters are placed next to the maximum weight for that load range.
Which type of tire to use: Tires are engineered specifically for different types of vehicles. Passenger car tires are designed to provide a soft ride and grip the road during turns and adverse weather. Light truck tires have stiffer sidewalls to carry heavier loads, but also are engineered for safe handling and road gripping ability. Trailer tires, on the other hand, are designed to give a soft ride and to slide sideways or scrub the road while cornering. Because of these differences, never put light truck tires on a trailer. Some people think that if the tire is good enough for a truck, it must be good enough for a trailer, but this is a fallacy. Light truck tires are not engineered for the unique stresses of trailering.