Tire Code Translation
Here is a brief description of the tire codes that will be found on the sidewall of a modern tire.
Section Width The cross sectional width of the tire in millimeters from sidewall to sidewall. Generally, the wider the section rating, the wider the tread width and contact patch (rubber in contact with the road), but there is quite a bit of variability between makes of tires. One brand's 195 section tires may have more tread width than another brand's 205s.
Tire Cross Section
Aspect Ratio The percent ratio of the tires cross sectional height (bead to tread) to section width. Also known as the tire profile. A 50 series tire will have a height of one-half the setion width.
Construction R = Radial ply, B = Bias ply-belted, D = Bias ply.
Rim Size Diameter of the rim (in inches) which this tire was designed to fit. Unfortunately, there is no indicator of what rim width the tire will fit; best consult a knowledgeable source here, but the rule of thumb is:
rim width = 90% of tread width plus or minus 10% (so running 345/30R13s on stock 4.5 inch wide Spitfire rims is NOT recommended).
Load Index A code number for those who can't read the smaller print of the maximum load limit (see below) but do like to look up numbers on charts.
Speed Rating Letter code for the maximum sustained speed that the tire is designed for:
Q = 162 kph / 99 mph
S = 184 kph / 112 mph
T = 193 kph / 118 mph
U = 203 mph / 124 mph
H = 213 kph / 130 mph
UTUG - Uniform Tire Quality Grading Indicates the following three factors:
Treadwear - higher numbers generally indicate harder, longer wearing compounds. As an example fairly hard compound would be a 480, a fairly soft street compound would be a 250, and a DOT "street" race tire would be around 60. This number seems to be a better indication of grip (at least under dry pavement conditions) than the traction rating below. As with many things in life, there is a tradeoff between treadwear and grip; those gumball slalom tires with the stick and consistancy of marshmallows will let you pull +1G turns but should have a wear life rated in meters instead of kilometers. On the other hand those iron hard shoes with the 300,000 km treadwear warranty could lead to unexpected thrills if you stuff your pride and joy into a corner just a wee bit too fast.
Traction - A highly imaginary A, B, C traction grade - anything with slightly more grip than a banana peel seems to get an A. Actually a controlled test of wet stopping ability.
Temperature - Another A, B, C scale, this time of the tires resistance to heat generation. Especially important for tow vehicles, trailers and other beasts of burden.
Load and Pressure limits Maximum load (per tire) in pounds and maxiumum inflation pressure (cold) in pounds per square inch.
DOT Compliance Codes Indicates the manufacturer, factory, tire size, production line, production date and european quality standards codes. Useful for product recalls and class action suits.
Additional Markings The following codes may also be present:
Application code - This code appears in front of the tire size. It designates the vehicle type this tire was designed for:
M + S - Mud and Snow tire.
Arrow on sidewall - indicates a unidirectional tread, designed for use in the direction of rotation pointed by the arrow.