Sizing
How to Read Your Tire Sidewall: Decoding LT255/75R17 (and Why It Matters for Tire Cover Sizing)
Every tire has a code molded into the sidewall that contains everything you need to know about its dimensions. Most owners have never looked at it closely, and that is fine until you need to order something specifically sized to the tire — a spare cover, a snow chain set, or a replacement at a tire shop that does not have your record on file.
The code looks like alphabet soup at first glance — “LT255/75R17” or “33×12.50R15” — but it follows a simple structure once you know what each part means. Decoding it gets you a cover size in about ten seconds, without measuring anything.
There are two formats in common use. Metric (LT255/75R17) is the standard on most modern passenger vehicles, SUVs, and light trucks. Imperial (33×12.50R15) shows up most often on off-road tires and is, ironically, the easier of the two to read.
Here is how to read both, with worked examples for the Wrangler, G-Wagon, and a few other common vehicles.
The tire-size code on your sidewall (for example, LT255/75R17) tells you everything you need to pick the right spare tire cover. Decoded: LT = light truck, 255 = section width in mm, 75 = aspect ratio (sidewall height as a percentage of width), R = radial, 17 = rim diameter in inches. Total tire diameter for that code: about 32 inches, meaning a 32″ cover.
The two common tire-code formats
Tires use one of two notation systems.
Metric format: LT255/75R17
- LT = Light Truck construction (built for trucks and SUVs).
- P = Passenger car construction.
- ST = Special Trailer construction.
- 255 = Section width in millimeters (the tread width).
- 75 = Aspect ratio (sidewall height as a percentage of width). 75% of 255 mm = 191 mm sidewall.
- R = Radial construction (essentially every modern tire).
- 17 = Rim diameter in inches.
Imperial format: 33×12.50R17
- 33 = Overall tire diameter in inches (the easy one).
- 12.50 = Section width in inches.
- R = Radial.
- 17 = Rim diameter.
The imperial format is easier because the first number is already your cover size. Metric needs a small calculation.
Calculating cover size from the metric format
The formula:
diameter_inches = rim_inches + 2 × (width_mm × aspect / 100) / 25.4
Example 1: LT255/75R17 → 17 + 2 × (255 × 0.75 / 25.4) = 17 + 15.05 = 32.05 inches → 32″ cover.
Example 2: 265/60R18 (G-Wagon stock) → 18 + 2 × (265 × 0.60 / 25.4) = 18 + 12.52 = 30.52 inches. The G-Wagon’s stock cover is sized 33″ rather than 31″ due to mounting clearance on the tailgate carrier.
Example 3: LT285/70R17 (Wrangler Rubicon) → 17 + 2 × (285 × 0.70 / 25.4) = 17 + 15.71 = 32.71 inches → 33″ cover.
Where to find the size code
Two places:
- The tire sidewall. The code is molded into the rubber, usually visible on the spare. Wipe with a damp cloth if mud or grime obscures it.
- The driver-side door jamb. The OEM-recommended tire size is printed on the door jamb sticker. This may not match your spare if you have upgraded tires aftermarket.
If the sidewall is unreadable from age or damage, fall back to a tape measure. Edge of tread to edge of tread through the center equals your cover size in inches.
Frequently asked questions
My sidewall says “33×12.50R15.” Cover size?
33″. Imperial-format codes start with the overall diameter in inches. The first number is your cover size.
My sidewall says “235/85R16.” Cover size?
31.7″, rounding to a 32″ cover. Calculation: 16 + 2 × (235 × 0.85 / 25.4) = 16 + 15.7 = 31.7 inches.
What does the LT prefix mean for cover sizing?
Nothing for sizing. LT (Light Truck) is a construction designation that describes how the tire is built, not its dimensions. The cover size is determined by the numbers (255/75R17), not the LT prefix.
I have run-flat tires. Same sizing approach?
Yes. Run-flat construction does not change the overall tire diameter. Use the same formula or chart.
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