How to Read a Tire Sidewall: Every Number, Decoded

Before you buy a tire, you have to speak its language. The good news: the whole code is stamped right on the sidewall, and once you know how to read it, ordering the correct tire takes about ten seconds. Let's decode a common example: 225/45R18 91V.

225 — Section width (mm)

The width of the tire in millimetres, from sidewall to sidewall. Bigger number, wider tire. This has to match what your vehicle (and your wheels) are designed for.

45 — Aspect ratio (%)

The sidewall height as a percentage of the width. A 45 here means the sidewall is 45% of 225mm. Lower numbers mean a shorter, stiffer sidewall (sportier, sharper steering, harsher ride); higher numbers mean more cushion.

R — Construction

"R" means radial — the construction used by virtually every modern passenger tire. You will occasionally see "ZR" on high-performance tires, which just flags a very high speed rating.

18 — Rim diameter (in)

The wheel diameter, in inches, that the tire is built to mount on. An 18 here fits an 18" wheel — and only an 18" wheel. This one is non-negotiable.

91V — Load index & speed rating

The 91 is the load index (how much weight the tire can carry — 91 is about 615 kg per tire). The letter V is the speed rating (V = up to 240 km/h). Always meet or exceed your vehicle's original spec; going lower on load rating is unsafe.

Two more things worth finding

• The DOT date code: the last four digits of the DOT stamp are the week and year of manufacture (e.g., 2624 = the 26th week of 2024). Fresh is better.
• The UTQG rating (e.g., 400/A/A): treadwear, traction, and temperature grades — handy for comparing longevity between two similar tires.

Still not sure your size is right? Every listing on TheAutoRack shows the full size breakdown, and our fitment search lets you punch in your exact numbers. If something looks off, we double-check before it ships.