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Size guide

Reading your tire size.

The size is molded into the tire wall, in a form like 255/50R20. Here is what each part of it means, where else to find it, and what to do when the wall is too worn to read.

The four parts of a tire sizeA tire seen from the side with 255/50R20 moulded on the wall. 255 — width across the tread, in millimeters. 50 — sidewall height, 50% of the width. R — radial construction, like nearly every road tire. 20 — wheel diameter, in inches.255/50R20255 — width across the tread, in millimeters50 — sidewall height, 50% of the widthR — radial construction, like nearly every road tire20 — wheel diameter, in inches

Where to look

Three places carry the same size. The tire wall is the one that is definitely right for the tires actually on the car — a placard tells you what the manufacturer fitted, which is not always what is on it now.

The size 255/50R20 molded into a tire's sidewall, above the wheel rim.
The tire wall.
A tire and loading information placard inside a driver's door frame, listing the front and rear tire size 255/50R20.
The door-jamb placard.
  1. The tire wall. Molded into the rubber on the outward face, usually the largest marking on it.
  2. The door-jamb placard. Open the driver’s door and look at the frame. It lists the sizes the vehicle left the factory with.
  3. The owner’s manual. Same information as the placard, in the tires section.

Read it with us

In 255/50R20: 255 is the width in millimeters, measured across the tread. 50 is the sidewall height as a percentage of that width — so a 50 on a 255 is about 128 mm of rubber between the wheel and the road. R is radial construction, which nearly every road tire is. 20 is the wheel diameter in inches.

Some walls carry letters before the width: P for passenger, LT for light truck, ST for a special trailer tire, T for a temporary spare. Others use a different shape entirely — 31X10.50R15 is a flotation size, where the first number is the overall height in inches rather than a width in millimeters.

Same width, three sidewall heightsA tire in cross-section on its wheel. The tread stays 255 mm wide while the sidewall changes height: 35% of 255 mm ≈ 89 mm of sidewall; 50% of 255 mm ≈ 128 mm of sidewall; 70% of 255 mm ≈ 179 mm of sidewall.255/35R2035% of 255 mm ≈ 89 mm of sidewall255/50R2050% of 255 mm ≈ 128 mm of sidewall255/70R2070% of 255 mm ≈ 179 mm of sidewall20″ wheel255 mm across
Same width, three sidewall heights.
Or try:
Where the size is written on a tireA tire seen from the side, showing 255/50R20 on the sidewall. Select a part of the size to see what it means.255/50R20
255
255 mm wide, tread edge to tread edge
50
Sidewall is 50% of the width — so about 128 mm tall
R
Radial — the belts run across the tire
20
Fits a 20-inch wheel

The rest of the wall

After the size come two more markings: a number and a letter, like 103V. The number is the load index — the most weight that tire is built to carry. The letter is the speed rating.

Load index, in part
IndexPer tire
911,356 lb
951,521 lb
991,709 lb
1031,929 lb
1102,337 lb
1213,297 lb
Speed rating, in part
LetterUp to
S112 mph
T118 mph
H130 mph
V149 mph
W168 mph
Y186 mph

The DOT code

Somewhere on the wall is DOT followed by a string of characters. The last four digits are the only part most people need: a week and a year.

Reading the DOT date codeThe last four digits of the DOT code are the week and the year the tire was built — 3223 means the 32nd week of 2023.DOTU2LL LMLR3223Week 32 of 2023The first characters identify the plant and the build.The last four are the date.

Rubber ages whether or not it is driven on. A tire past about six years is worth having looked at, even with plenty of tread left — not because it is necessarily unsafe, but because age is not something tread depth tells you.

Not sure? Three ways we make it easy

You do not need to know your size to book. Pick it from the numbers below, photograph the wall and let us try to read it, or skip it entirely at the booking step — your applicator confirms the size on the day either way.

A phone held up to a tire's sidewall, its camera view showing the size 255/50R20.

255/50R20 = width 255 · sidewall 50 · wheel 20. Reads differently?

Where the size is written on a tireA tire seen from the side, with the size moulded on the sidewall.

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