Size and Shape / Speed Rating
/ Tread Wear Rating
Traction and Temperature / Date of Manufacture
Maximum Tire Pressure
Typically, the tire size looks something like P185/70R14. The "P"
means a passenger-car or light-truck tire; "LT" means
a tire for light-truck use only. (Some tires omit that.) The next
three digits tell you how wide the tire is, in millimeters (here
Large and high-performance cars tend to have wide tires,
greater than 200 mm. The two digits after the slash are the "aspect
ratio" - the ratio of the height of the sidewall to the tire
width. Here, the sidewall height is 70 percent of the width, or
about 130 mm. Performance tires tend to have a low aspect ratio
- usually 60 or less - and a squat appearance.
simply means radial ply - a design used for virtually all tires.
The two digits at the end are the diameter of the wheel, in inches.
Some tires have an additional letter either with the size designation,
before the "R" (185/70SR14) or just after the wheel diameter.
Such letter codes reflect the maximum speed the tire is certified
to sustain. Conventional tires may carry an S(112 mph) or T (118
mph) speed rating or none at all. Most performance tires carry an
H (130 mph); ultra-high-performance tires, a V (149mph), Z (149-plus),
W (168 mph) or Y (186 mph).
This is a measure of how well a tire's tread will wear, compared
with a "reference" tire graded at 100. Theoretically,
a tire with a wear index of 450 (relatively high) should last three
times a long as one with an index of 150 (relatively low). Tire
makers conduct these tests themselves.
Scores are for Government tests for stopping on a wet surface and
resisting the effects of high temperatures. Ranks are given as A,
B, or C. "A" is best, "C" is worst.
Every tire has a US Department of Transportation serial number
- something like DOT DBUA A44 414 GCD 415. The last three
digits tell you the week and year the tire was made; thus,
415 indicates that the 41st week of 1995.
The tires highest safe inflation pressure, in pounds per square
inch (for example, 35 PSI MAX PRESS). It's best to follow the inflation
recommendations in the car owner's manual. In any case, don't exceed
the maximum listed on the tire. Buy an inexpensive tire gauge to
keep in your glove compartment.