How to read a tire size
The string on the sidewall is a complete specification. Take 265/70R17 115T: width 265 mm, sidewall 70% of the width, radial construction, a 17-inch wheel, load index 115 and speed rating T. Here is every piece, in order.
The size itself
| Piece | Example | What it means |
|---|---|---|
| Service prefix | P / LT / ST / T | Passenger, light truck, special trailer or temporary spare. No prefix usually means a Euro-metric passenger tire. |
| Width | 265 | Tread width in millimetres, measured sidewall to sidewall. |
| Aspect ratio | 70 | Sidewall height as a percentage of the width. 70% of 265 mm is a 185.5 mm sidewall. Lower numbers mean a shorter, sportier sidewall. |
| Construction | R | Radial — the cords run across the tread. Nearly every modern road tire is radial; D (diagonal) and B (belted) survive on trailers and classics. |
| Wheel diameter | 17 | The diameter of the wheel the tire mounts on, in inches. This one never changes units. |
Those five pieces are everything a size comparison needs: from them you can work out the overall diameter (rim × 25.4 + 2 × sidewall), which is what the tire comparison calculator and the single-size calculator do.
Load index and speed rating
The two characters after the size — 115T in the example — are not part of the size. The load index is a code for the maximum weight one tire can carry: 115 corresponds to 1,235 kg (2,716 lb) per tire on the standard load index table. The speed letter is the maximum speed the tire is rated for under its rated load: T is 190 km/h (118 mph), H is 210 km/h, V is 240 km/h, and S — common on winter and truck tires — is 180 km/h. When you change sizes, the replacement should match or exceed the original load index, which is one reason a size swap is a job to confirm with the vehicle manual or a professional tire shop rather than by diameter alone.
Flotation sizes
Off-road and light-truck tires often skip the metric system entirely: 31x10.50R15 states a 31-inch overall diameter and a 10.50-inch width on a 15-inch wheel. The sidewall is whatever is left: (31 − 15) ÷ 2 = 8 inches. Nothing is missing — the diameter that metric sizes make you calculate is printed right there, which is why flotation and metric sizes convert into each other cleanly, as in the 235/75R15 vs 31x10.50R15 comparison.
The DOT date code
Elsewhere on the sidewall, after the letters DOT, a four-digit code gives the week and year the tire was made: 3223 means week 32 of 2023. Tire age matters independently of tread — most manufacturers suggest close inspection after five years and replacement by ten at the latest, whatever the tread looks like. Check the maker's own guidance for your tire.
Frequently asked questions
Where do I find my tire size?
On the tire's sidewall, in the driver's door jamb sticker, or in the vehicle manual. The door sticker lists the size the car left the factory with, which is the reference point for any comparison.
Does the aspect ratio change the diameter?
Yes — it is half the diameter equation. At the same 265 mm width, a 70-series tire has a 185.5 mm sidewall and a 60-series only 159 mm, so the 60-series tire is 53 mm smaller in diameter despite the identical width number.
Can a tire with a different load index be fitted?
The replacement should carry at least the load index of the original fitment. Going lower reduces the vehicle's rated carrying capacity. Speed rating should also meet the vehicle maker's specification. When in doubt, a professional tire shop will check both against the vehicle.