How do I read my tyre size?

A marking like 225/45R17 means: 225 = tyre width in mm, 45 = aspect ratio (sidewall height as % of width), R = radial construction, 17 = rim diameter in inches.

How much diameter difference is acceptable?

Most manufacturers recommend staying within 3% of the original tyre diameter. Under 1% is ideal — the speedo error will be negligible. Over 3% may cause speedo inaccuracy, clearance issues, and affect ABS/traction control calibration.

Why does tyre diameter affect the speedometer?

Your speedometer calculates speed based on wheel revolutions per distance. A larger tyre covers more ground per revolution, so the speedo reads lower than actual speed. A smaller tyre causes the speedo to read higher than actual.

Tyre Size Calculator — Decode & Compare Tyre Markings

Read the markings on your tyre sidewall — e.g. 225/45R17

Tyre A

Width (mm)

Aspect (%)

Rim (inch)

Tyre B — compare (optional)

Width (mm)

Aspect (%)

Rim (inch)

Tyre A

Tyre B

About the Tyre Size Calculator

Tyre size markings encode critical measurements in a standardised format. A marking like 205/55 R16 means: 205mm tread width, 55% aspect ratio (sidewall height = 55% of 205mm = 112.75mm), R = radial construction, 16-inch wheel diameter. Understanding these measurements is essential when choosing replacement tyres, fitting aftermarket wheels, or comparing sizes for fuel economy or handling changes.

Decoding the full tyre marking

Width (205) — tread width in mm from sidewall to sidewall
Aspect ratio (55) — sidewall height as a percentage of tread width. Lower = lower-profile tyre.
Rim diameter (16) — wheel rim diameter in inches
Load index (e.g. 91) — maximum load capacity (91 = 615 kg per tyre)
Speed rating (e.g. V) — maximum sustained speed (V = 240 km/h)

Why overall diameter matters

Fitting a tyre with a different overall diameter affects speedometer accuracy, ABS calibration, and ground clearance. A ±3% change in diameter is generally acceptable; beyond this, the speedometer should be recalibrated. Manufacturers specify an approved size range; staying within it is important for safety and legal compliance.

Seasonal tyre recommendations

Tyre choice significantly affects safety and running costs. Summer tyres perform best in dry and wet conditions above 7°C. Winter tyres use a different rubber compound that remains pliable below 7°C, improving grip on cold, wet, and snowy roads. All-season tyres offer a compromise suitable for climates with mild winters.

Summer tyres — best dry and wet grip above 7°C; standard for most UK cars
Winter tyres — softer compound improves grip below 7°C; mandatory in many Central and Northern European countries
All-season tyres — compromise between summer and winter performance; suitable for UK climate in most regions
M+S marking — Mud and Snow; basic winter capability; not as capable as tyres with the Three-Peak Mountain Snowflake (3PMSF) symbol

Frequently Asked Questions

What does 205/55 R16 mean on a tyre?

205 is the tread width in millimetres. 55 is the aspect ratio: sidewall height = 205 x 0.55 = 112.75mm. R indicates radial construction. 16 is the rim diameter in inches. Overall tyre diameter = (2 x sidewall height in mm) + (rim diameter x 25.4mm) = 225.5 + 406.4 = 631.9mm.

How do I calculate tyre diameter?

Sidewall height = tread width x (aspect ratio / 100). Overall diameter = (2 x sidewall height) + (rim diameter x 25.4). For 225/45 R17: sidewall = 225 x 0.45 = 101.25mm. Diameter = (2 x 101.25) + (17 x 25.4) = 202.5 + 431.8 = 634.3mm.

Can I fit a wider tyre on my car?

Wider tyres can be fitted if the wheel arch and suspension geometry allow it. Staying within plus or minus 20mm of original width is generally safe. Wider tyres improve dry grip but can increase aquaplaning risk in wet conditions and reduce fuel economy through higher rolling resistance.

What happens if I fit the wrong size tyre?

A different overall diameter affects speedometer accuracy (reads too high or too low), odometer readings, and ABS/traction control calibration which rely on wheel speed sensors. A 3% change in diameter is generally acceptable; beyond that, speedometer recalibration is recommended.

What are the load index and speed rating on a tyre?

The load index is a number indicating maximum load per tyre (e.g. 91 = 615 kg, 94 = 670 kg, 97 = 730 kg). The speed rating is a letter indicating maximum sustained speed (V = 240 km/h, W = 270 km/h, Y = 300 km/h). Always fit tyres meeting or exceeding your vehicle's minimum specified ratings.