Calculate tire sidewall, overall diameter, circumference, and revolutions per mile.
This tool provides estimates for informational purposes only. It is not a substitute for professional advice. Individual results vary based on your inputs and assumptions, so review important decisions with a qualified professional.
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Understanding tyre (tire) sizes is essential for anyone buying replacement tyres, upgrading wheel sizes, checking speedometer accuracy, or comparing vehicle specifications. The size code printed on every tyre's sidewall contains a wealth of engineering data, but it is written in a format that most drivers have never been taught to read. This comprehensive guide explains every element of the tyre size code for both UK and US drivers, covers legal requirements, and explains how size changes affect your vehicle.
The standard modern tyre size code uses this format: 225/45R17 91W
To calculate the overall tyre diameter:
Diameter (mm) = (Width x Aspect Ratio / 100 x 2) + (Rim diameter in inches x 25.4)
Example for 225/45R17: (225 x 45/100 x 2) + (17 x 25.4) = (225 x 0.45 x 2) + 431.8 = 202.5 + 431.8 = 634.3 mm total diameter.
Circumference = diameter x pi = 634.3 x 3.14159 = 1,992 mm (approximately 1.99 metres per revolution). This is important for speedometer calibration.
Your speedometer is calibrated to your original factory tyre size. Changing to a larger or smaller tyre diameter affects accuracy:
| Change | Effect on Speedometer | Effect on Odometer |
|---|---|---|
| Larger diameter (+3%) | Reads 3% lower than actual speed | Reads fewer miles than actual |
| Smaller diameter (-3%) | Reads 3% higher than actual speed | Reads more miles than actual |
UK law (Construction and Use Regulations) and US DOT regulations both require speedometers to read within acceptable tolerances. A 3% diameter change (roughly equivalent to going from 45 to 50 or 40 aspect ratio) is generally considered the acceptable limit before the speedometer requires recalibration or the change is legally questionable.
Plus sizing (also called upsizing) involves fitting larger diameter wheels with lower-profile tyres to maintain approximately the same overall tyre diameter. The goal is to keep speedometer accuracy while improving handling through a wider, stiffer tyre contact patch.
Example: Stock 205/60R15 (diameter 634mm) can be replaced with 225/50R16 (diameter 636mm, 0.3% different — acceptable) for Plus 1 sizing. The wider 225mm tyre in 50 profile on a 16" rim maintains approximately the same rolling diameter.
UK tyre law is strict and the penalties are severe:
The US has no federal minimum tread depth law, but most states use 2/32 inch (1.6 mm, same as UK) as the minimum. Many safety organisations recommend replacement at 4/32 inch. Key US tyre requirements:
| Load Index | Max Load (kg) | Speed Rating | Max Speed |
|---|---|---|---|
| 80 | 450 kg | T | 190 km/h (118 mph) |
| 85 | 515 kg | H | 210 km/h (130 mph) |
| 91 | 615 kg | V | 240 km/h (149 mph) |
| 95 | 690 kg | W | 270 km/h (168 mph) |
| 100 | 800 kg | Y | 300 km/h (186 mph) |
You must never fit a tyre with a lower load index or lower speed rating than specified by the vehicle manufacturer. Fitting a higher-rated tyre than specified is permitted and sometimes beneficial (e.g., fitting V-rated tyres on a car that only requires H-rated for added safety margin).
Tyre compound and tread pattern choice varies dramatically by season, particularly in the UK and northern US states:
TPMS has been mandatory on all new EU-type approved vehicles since November 2014, and in the UK (post-Brexit) this requirement was retained. In the US, TPMS has been mandatory since 2008 (FMVSS 138). TPMS warns when tyre pressure drops 25% or more below the recommended level.
Correct tyre pressure is critical: under-inflation by 20% increases rolling resistance by approximately 3%, reduces fuel economy, increases tyre wear, and raises the risk of tyre failure. UK recommended tyre pressures are listed in the vehicle handbook and typically on a sticker inside the fuel filler flap or driver's door jamb.
EU tyre labelling (mandatory in the UK post-Brexit under Regulation (EU) 2020/740) rates tyres from A to E for rolling resistance, wet grip, and external rolling noise. An A-rated rolling resistance tyre can improve fuel economy by up to 7% compared to a G-rated tyre. For UK and EU vehicles, this label appears on all new tyres sold and must be displayed at point of sale.
225 is the tyre width in millimetres. 45 is the aspect ratio — the sidewall height is 45% of the width (101 mm). R means radial construction. 17 is the wheel rim diameter in inches. Any numbers after (like 91W) are load index and speed rating.
The UK legal minimum tread depth is 1.6 mm across the central three-quarters of the tyre's width around the full circumference. The penalty for each illegal tyre is up to £2,500 and 3 penalty points. Many safety experts recommend replacing tyres at 3 mm (summer) or 4 mm (winter) for adequate wet-weather safety margins.
Diameter (mm) = (Width x Aspect Ratio / 100 x 2) + (Rim diameter in inches x 25.4). For 225/45R17: (225 x 0.45 x 2) + (17 x 25.4) = 202.5 + 431.8 = 634.3 mm total diameter.
Larger overall diameter causes the speedometer to read lower than actual speed (the tyre covers more ground per revolution than the calibration assumes). It also raises the vehicle's ride height slightly and may cause rubbing against wheel arches or suspension components. A 3% diameter increase is typically the maximum for acceptable speedometer accuracy.
Winter tyres are not legally required in the UK (unlike Germany, Austria, and Scandinavia) but are strongly recommended for temperatures consistently below 7 C. Winter tyres outperform summer tyres in cold, wet, and icy conditions even when there is no snow, because the softer compound maintains grip at low temperatures. All-season tyres offer a practical compromise for UK drivers facing occasional winter conditions.
TPMS (Tyre Pressure Monitoring System) monitors tyre pressures and warns when any tyre drops 25% or more below recommended pressure. It has been mandatory on all new EU/UK type-approved vehicles since November 2014, and on all new US vehicles since 2008. A TPMS warning light means you must check and correct tyre pressures promptly.
The speed rating letter on a tyre indicates the maximum sustained speed it is designed for: T=190 km/h, H=210 km/h, V=240 km/h, W=270 km/h, Y=300 km/h. You must never fit a tyre with a lower speed rating than specified for your vehicle. Higher-rated tyres are permitted and may provide better handling at legal speeds.
Plus sizing means fitting a larger diameter wheel (in 1-inch increments) with a lower profile tyre to maintain approximately the same overall rolling diameter. Plus 1 sizing adds 1 inch to the rim and reduces aspect ratio accordingly. Benefits include wider tyres for better handling; drawbacks include stiffer ride and greater susceptibility to pothole damage on low-profile tyres.