How to read tire size numbers
Tire sizes look like a random string until you know that they cram three different unit systems into one code. Here's how to decode any of them in under a minute.
The anatomy of 265/70R17 112T
Reading left to right, each piece answers a different question:
| Part | Means | In this example |
|---|---|---|
| 265 | Section width, millimetres | 265mm (~10.4") wide |
| 70 | Aspect ratio: sidewall as % of width | sidewall = 70% of 265 = 185.5mm |
| R | Construction (R = radial) | radial |
| 17 | Wheel diameter, inches | fits a 17" wheel |
| 112 | Load index | ~2,469 lbs per tire |
| T | Speed rating | up to 118 mph |
Finding the real overall height
The number people actually want — how tall the tire is — isn't printed. You build it from the others:
- Sidewall height = width × aspect ÷ 100 = 265 × 70 ÷ 100 = 185.5mm
- Convert to inches = 185.5 ÷ 25.4 = 7.3"
- Overall diameter = wheel + 2 × sidewall = 17 + 2(7.3) = 31.6"
The mm → inches converter does this instantly, but it's worth understanding so you can sanity-check any size in your head.
Why this matters before you buy
Two sizes that look similar on paper can behave very differently. A 10mm width bump changes how the tire fills the wheel well and whether it fits your rims. A change in aspect ratio or wheel size changes overall diameter, which moves your speedometer, odometer, and effective gearing. Before committing to a new size, run it through the comparison calculator to see exactly what changes, and the rim fitment calculator to confirm the width suits your wheels.
Frequently asked questions
- What do the numbers on a tire mean?
- In a size like 265/70R17, 265 is the section width in millimetres, 70 is the aspect ratio (sidewall height as a percent of width), R is radial construction, and 17 is the wheel diameter in inches. Trailing characters are the load index and speed rating.
- Which tire number is the height?
- There is no direct height number — you calculate it. Multiply the width by the aspect ratio for the sidewall height, then add it twice to the wheel diameter. The aspect ratio (middle number) is the lever that controls height.
- What is the load index and speed rating?
- The two characters after the size, e.g. 112T, are the load index (112 = ~2,469 lbs per tire) and speed rating (T = 118 mph). Always match or exceed your vehicle’s original ratings.