How to read a tire size
What every number on your tire sidewall means — width, aspect ratio, construction, wheel diameter, load index and speed rating — with worked examples.
Every tyre has its full specification moulded into the sidewall, but it looks like a random string until you know the pattern. Take a common size like 195/65 R15 91H — here is exactly what each part means.
The core size: 195/65 R15
- 195 — the section width in millimetres, measured across the widest part of the tyre.
- 65 — the aspect ratio: the sidewall height as a percentage of the width. Here the sidewall is 65% of 195 mm ≈ 127 mm tall. A lower number (e.g. 40) means a shorter, sportier sidewall.
- R — radial construction, which is what virtually every modern car tyre uses.
- 15 — the wheel diameter in inches the tyre is designed to fit.
The service description: 91H
After the size you will usually see a number and a letter:
- 91 — the load index. 91 corresponds to 615 kg per tyre. It is a coded value, not kilograms directly — always look it up rather than comparing the numbers as if they were weights.
- H — the speed rating, the maximum speed the tyre is certified for. H is 210 km/h (130 mph); common ratings run T (190), H (210), V (240), W (270), Y (300).
Why overall diameter matters
When you change tyre size, the number that really matters is the overall diameter, because it drives your speedometer reading, effective gearing and how the tyre fits in the arch. Overall diameter is the wheel diameter plus two sidewalls:
diameter = wheel Ø + 2 × (width × aspect ÷ 100)
For 195/65 R15 that is 15 inches (381 mm) + 2 × 127 mm ≈ 635 mm. Change the size and if the new diameter differs, your speedometer will read high or low by the same percentage.
Keeping the same rolling diameter ("plus sizing")
Fitting a bigger wheel without changing your gearing or speedo means dropping the aspect ratio to keep the overall diameter the same — for example going from 195/65 R15 to 205/50 R16 or 215/45 R17. Each step up in wheel size drops the sidewall to compensate. Aim to keep the new diameter within about ±2–3% of standard so the speedometer stays accurate and the tyre still clears.
Quick reference
Reading left to right: width (mm) / aspect (% of width) construction wheel (inch) load index speed rating. Once you can decode one, you can decode any of them.
FAQ
Is a bigger tyre width always better?
A wider tyre can add grip but also weight, cost, road noise and tramlining, and it may rub. It also changes the overall diameter unless you drop the aspect ratio to match. Stay close to the factory rolling diameter and confirm the width clears your arches and suspension.
What does the aspect ratio actually change?
It sets sidewall height. A taller sidewall (higher number) rides softer and protects the wheel; a shorter one (lower number) sharpens steering response but rides firmer and is easier to damage on potholes.
Will a different tyre size mess up my speedometer?
Only if it changes the overall diameter. A tyre with a larger diameter makes the speedometer read lower than your true speed, and a smaller one reads higher, by the same percentage as the diameter change.