Tyre Size Calculator Guide - Understanding Tyre Dimensions UK

Tyre sizes can seem confusing, but understanding the numbers helps you choose the right tyres. This guide explains what each marking means.

Reading Tyre Markings

Calculating Tyre Dimensions

Speed Ratings

Load Index

Finding Equivalent Sizes

Alternative sizes should have a similar overall diameter (within 3%):

EU Tyre Labels

• Fuel efficiency: A (best) to E (worst)
• Wet grip: A (best) to E (worst)
• External noise: Decibels and class (A/B/C)
• Snow grip: 3PMSF symbol if applicable
• Ice grip: Ice symbol if applicable

Methodology: Understanding Tyre Dimension Calculations

Tyre sizing uses a combination of metric and imperial measurements that can be confusing at first glance. Understanding the underlying mathematics is essential for anyone comparing alternative sizes, checking speedometer accuracy after a wheel upgrade, or converting between European and American tyre designations.

Overall Diameter Calculation

The overall tyre diameter is the most critical measurement when comparing sizes, because it directly affects speedometer accuracy, ride height, and gearing. The formula is:

For a 225/45 R17 tyre: Sidewall = 225 x 0.45 = 101.25 mm. Wheel diameter = 17 x 25.4 = 431.8 mm. Overall diameter = (2 x 101.25) + 431.8 = 634.3 mm. This gives a rolling circumference of approximately 1,993 mm (diameter x pi), meaning the tyre completes about 502 revolutions per kilometre.

Speedometer Error from Size Changes

When you fit tyres with a different overall diameter, your speedometer will read incorrectly because it is calibrated to the original tyre's revolutions per kilometre. The error percentage equals the percentage change in overall diameter. If you move from a 631.9 mm diameter to a 645 mm diameter (a 2.1% increase), your speedometer will under-read by 2.1% - so when it shows 70 mph, your actual speed is approximately 71.5 mph. This has both legal and safety implications.

The 3% Rule

Most tyre professionals and vehicle manufacturers recommend that alternative sizes should be within 3% of the original overall diameter. Beyond this threshold, you may experience issues with ABS calibration, traction control behaviour, clearance problems inside the wheel arch, and inaccurate odometer readings that could affect warranty mileage claims or lease return mileage.

Plus-Sizing: Going Up in Rim Diameter

Plus-sizing involves fitting larger diameter wheels with lower-profile tyres to maintain approximately the same overall diameter. The standard conversions are: Plus 1 (e.g., 16 to 17 inch) reduces aspect ratio by about 10 points and increases width by 10 mm. Plus 2 (e.g., 16 to 18 inch) reduces aspect ratio by about 20 points and increases width by 20 mm. The trade-off is improved handling response and aesthetics versus a firmer ride and increased susceptibility to pothole damage.

UK-Specific Tyre Regulations and Context

UK tyre law is among the most strictly enforced in Europe, and understanding the regulations is essential for any driver. The consequences of non-compliance are severe: up to three penalty points and a fine of up to £2,500 per illegal tyre.

MOT Tyre Requirements

During the annual MOT test (required for vehicles over 3 years old), tyres are inspected for: tread depth (minimum 1.6 mm across the central three-quarters of the tread width, around the entire circumference), visible damage (cuts, bulges, exposed cord, cracking), correct fitting (suitable size and rating for the vehicle), and matching on the same axle (mixing radial and cross-ply on the same axle is illegal). Winter tyres and all-season tyres are permitted year-round in the UK and are not penalised at MOT.

UK Road Conditions and Tyre Choice

UK roads present specific challenges that influence tyre selection. The prevalence of potholes (the RAC reported over 29,000 pothole-related breakdowns in 2023) makes lower-profile tyres riskier, as there is less sidewall to absorb impacts. The UK's wet climate means that wet grip performance (shown on the EU tyre label as grades A to E) is arguably more important than dry grip for everyday driving. Run-flat tyres, once rare, are increasingly common on UK-market BMWs and Minis, though they are typically 20-40% more expensive than conventional tyres of the same size.

Seasonal Tyres in the UK

Unlike many European countries, the UK has no legal requirement for winter tyres. However, their use is growing: sales have increased significantly over the past decade, particularly in Scotland and northern England. Winter tyres (marked with the 3PMSF snowflake symbol) outperform summer tyres at temperatures below 7 degrees Celsius, which in much of the UK means they are beneficial from November to March. All-season tyres offer a compromise, performing adequately in both warm and cold conditions, though they do not match the performance of dedicated seasonal tyres at either extreme.

Tyre Labelling Post-Brexit

Since leaving the EU, the UK has retained the EU tyre labelling system but under UK law (The Tyre Labelling (Amendment) (EU Exit) Regulations 2020). The label rates fuel efficiency, wet grip, and external noise, and since 2021 also includes snow and ice grip information. Retailers are legally required to display these labels, making it easier for UK consumers to compare products objectively.

Worked Examples

Common Mistakes and Tips

Frequently Asked Questions

Can I fit different sized tyres on the front and rear of my car?

Some vehicles are designed with staggered fitments (wider rear tyres), particularly sports cars and performance vehicles. This is acceptable provided both sizes are listed in the manufacturer's specifications. However, fitting different sizes on your own initiative without manufacturer approval can affect handling, void insurance, and potentially cause MOT failure. Always consult your vehicle handbook before fitting non-standard sizes.

What does the DOT number on a tyre mean?

The DOT (Department of Transportation) code includes the tyre's date of manufacture as a four-digit number at the end. For example, "2423" means the tyre was made in week 24 of 2023. UK tyre retailers should not sell tyres more than 5-6 years old as new, and tyres over 10 years old should be replaced regardless of tread depth, as rubber degrades over time even when unused.

Are retreaded tyres legal in the UK?

Yes, retreaded tyres are legal for passenger vehicles in the UK, provided they comply with British Standard BS AU 144e and carry the appropriate markings. They are significantly cheaper than new tyres (typically 30-50% less) and are widely used on commercial vehicles. However, they are not permitted on the front axle of vehicles designed to carry more than 8 passengers or on vehicles with a maximum speed above 100 mph.

How much do tyre fitting and balancing cost in the UK?

Fitting charges typically range from £8-15 per tyre for standard passenger car sizes, with balancing adding another £5-10 per wheel. Mobile tyre fitting services (which come to your home or workplace) usually charge a call-out fee of £20-30 on top of per-tyre fitting charges. Valve replacement is often an additional £2-5 per tyre and is recommended each time tyres are changed.

UK Tyre Prices 2025

Prices per tyre, fitted. Prices vary by retailer and brand.

UK MOT Tyre Requirements

• Tread depth: Minimum 1.6mm across central 3/4 of tread width
• Condition: No cuts, bulges, or exposed cords
• Matching: Same size on each axle recommended
• Speed rating: Must meet or exceed vehicle requirements

Popular UK Tyre Brands

James Mitchell, ACCA

Chartered Accountant & Former HMRC Advisor

James is a Chartered Certified Accountant (ACCA) specialising in UK personal taxation and financial planning. With over 12 years in practice and a background as a former HMRC compliance officer, he brings authoritative insight to complex tax topics.