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, prior to 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
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 important 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
in order 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.