Reading a tyre sidewall: three unit systems, six characters

Six characters, three conventions

• 225 — section width in millimetres.
• 45 — aspect ratio: sidewall height as a percentage of the width, so 101 mm here.
• R — radial construction, which is virtually every road tyre made today.
• 17 — rim diameter in inches, equal to 431.8 mm.

Why the mixture exists

Wheel diameters were standardised in inches long before tyre construction moved to metric widths. Both conventions were entrenched by the time anyone considered harmonising them, so each stayed where it was strongest.

Overall diameter, and the two per cent rule

Add the rim diameter to twice the sidewall height: 431.8 + (2 × 101) = 633.8 mm. Change that total by more than about two per cent and your speedometer reads wrong, because it counts wheel rotations and assumes a fixed circumference. On modern cars the stability and traction systems use the same figure.

The characters after the size

A code such as 91V is not decoration. The number is a load index — 91 means 615 kg per tyre — and the letter caps speed, with V allowing 149 mph. Fitting a lower rating than the manufacturer specifies can invalidate insurance.

Before you order

Check the approved sizes in the handbook or on the door pillar sticker, convert the rim figure if you are comparing metric specifications, and confirm the load and speed codes match or exceed the original.

The rest of the sidewall

The size code is only part of what is moulded into the rubber. Four other markings matter when you buy:

• Load index. A two- or three-digit number — 91 means 615 kg per tyre. Fitting below the manufacturer's figure is an MOT failure in the UK and can void insurance.
• Speed rating. A letter: T is 190 km/h, H is 210, V is 240, W is 270. It must meet or exceed the vehicle's specification.
• DOT date code. Four digits giving week and year of manufacture. "2325" means the twenty-third week of 2025. Rubber ages even unused, and many manufacturers advise replacement at ten years regardless of tread.
• Tread wear indicators. Small raised bars in the grooves at the legal minimum depth, 1.6 mm across most of Europe.

Calculating rolling circumference

Overall diameter determines how far the car travels per wheel revolution, which is what the speedometer and odometer are calibrated against. For 225/45 R17:

1. Sidewall height: 225 × 0.45 = 101.25 mm
2. Rim diameter: 17 × 25.4 = 431.8 mm
3. Overall diameter: 431.8 + (2 × 101.25) = 634.3 mm
4. Circumference: 634.3 × π = 1,992 mm

Fit 225/50 R17 instead and the sidewall grows to 112.5 mm, the diameter to 656.8 mm — 3.5 per cent larger. The speedometer will read about 3.5 per cent low, and the odometer will under-record by the same margin.

Plus sizing without breaking anything

Going up a rim size while keeping the overall diameter roughly constant is called plus sizing. Each inch of rim diameter added needs the aspect ratio reduced by roughly 10 points to compensate:

The first two are interchangeable in diameter terms. The third is at the edge of acceptable and will noticeably firm up the ride, because there is less sidewall to absorb anything.

Why speedometers read high from the factory

Regulations require that a speedometer never under-reads, so manufacturers calibrate them to read a few per cent high. That is why a GPS speed of 68 mph shows as 70 on the dial. A larger tyre eats into that safety margin and can push the reading below true speed — which is the direction the law does not allow.

The takeaway

Read all the markings, not just the size. Convert the rim to millimetres, calculate overall diameter, and stay within two per cent of standard.