How to Read a Tire Size

Every tire carries its full specification on the sidewall, and once you can read it the whole category becomes much easier to shop. Take 275/65R18 116T as the example and work through it piece by piece.

The metric format, piece by piece

Element	Example	What it means
Section width	275	Width of the tire in millimeters at its widest point, sidewall to sidewall
Aspect ratio	65	Sidewall height as a percentage of section width. Here 65% of 275mm is about 179mm
Construction	R	Radial. Almost every modern tire is radial. B means bias belted, D means diagonal bias
Wheel diameter	18	Diameter of the wheel the tire mounts to, in inches
Load index	116	A coded number for maximum load capacity per tire. 116 is 2,756 lb
Speed rating	T	Maximum sustained speed the tire is rated for. T is 118 mph

A lower aspect ratio means a shorter sidewall. A 275/65R18 has a much taller sidewall than a 275/40R18, which is why the first looks like a truck tire and the second looks like a sports car tire even though the width and wheel size are identical.

To get overall diameter from a metric size: sidewall height in inches is (width in mm x aspect ratio / 100) / 25.4. Double it, because there is sidewall above and below the wheel, then add the wheel diameter. For 275/65R18 that is (275 x 0.65) / 25.4 = 7.04 inches of sidewall, doubled to 14.08, plus 18 = 32.1 inches overall.

Load index

Load index is a coded number, not a weight. Each number maps to a specific capacity per tire, and you need enough capacity that four tires comfortably exceed your gross vehicle weight rating.

Load index	Capacity per tire	Load index	Capacity per tire
91	1,356 lb	112	2,469 lb
95	1,521 lb	114	2,601 lb
99	1,709 lb	116	2,756 lb
103	1,929 lb	118	2,910 lb
105	2,039 lb	120	3,086 lb
108	2,205 lb	121	3,197 lb
110	2,337 lb	126	3,748 lb

Speed rating

Speed rating is the maximum sustained speed the tire is built to handle at full load. It is not a suggestion to drive that fast, it is a construction and heat-tolerance specification. Fitting a lower speed rating than the factory specification can affect handling and, in some jurisdictions, insurance.

Code	Max speed	Typical use
Q	99 mph	Winter tires, some light truck
S	112 mph	Standard passenger and light truck
T	118 mph	Common on trucks, SUVs and touring tires
H	130 mph	Performance touring, many sedans
V	149 mph	Sport and performance
W	168 mph	Ultra high performance
Y	186 mph	Max performance and supercars

P-metric, LT and XL prefixes

• No prefix or P: passenger tire, sometimes called P-metric. Built for cars, crossovers and light-duty truck use. Softer ride, lower load capacity.
• LT: light truck construction. More plies, stiffer sidewall, holds higher pressure and carries more weight. The right choice if you tow, haul or air down off-road.
• XL or Extra Load: a passenger tire reinforced to carry more than the standard version of the same size. Requires higher inflation pressure to reach its rated capacity.
• ST: special trailer. Very stiff sidewalls for sway resistance. Never fit these to a powered or steered axle.

One important note: an LT tire in the same nominal size as a P-metric tire will usually have a lower load capacity at the same pressure, and it reaches its higher rating by running higher pressure. Always inflate to the vehicle placard for P-metric, and to the load table for LT.

Load range

Load range	Ply rating	Typical max pressure	Common application
C	6-ply	50 psi	Light utility, small trailers, some half-ton
D	8-ply	65 psi	Half-ton towing, mid-size off-road
E	10-ply	80 psi	Three-quarter and one-ton trucks, heavy towing
F	12-ply	95 psi	One-ton dually, heavy commercial

Flotation sizing

Off-road tires are often sold in the flotation format instead: 35x12.50R17. Here the first number is the overall diameter in inches, the second is the section width in inches, and the last is the wheel diameter. This format is easier to shop for lift and clearance because the diameter is stated directly rather than calculated.

Converting between the two formats is the most common tire-shopping math. Our tire size conversion chart lists the common equivalents.

The DOT date code

Every tire has a DOT code on the sidewall ending in four digits. The first two are the week of manufacture and the last two are the year, so 3224 means the 32nd week of 2024. Rubber ages whether or not the tire is used, and most manufacturers recommend replacement at 6 to 10 years regardless of tread depth. For trailer tires the window is shorter, typically 3 to 5 years.

Check this when buying. A new tire that has sat in a warehouse for three years is not the same product as one made last month.

UTQG treadwear, traction and temperature

UTQG stands for Uniform Tire Quality Grading and appears as a treadwear number plus two letter grades. Treadwear is a comparative number where higher means longer expected life, but it is self-reported by the manufacturer, so it is most meaningful when comparing tires within one brand. Traction is graded AA, A, B or C based on straight-line wet braking. Temperature is graded A, B or C for heat dissipation.