Tyre Size Calculator
Compare two tyre sizes and see the difference in sidewall height, rolling diameter, circumference, revolutions per mile, speedometer reading, ride height and wheel-arch clearance. This is useful when checking replacement tyres, upsizing wheels, changing profile, or comparing OEM and aftermarket tyre sizes.
Compare tyre sizes
Enter the original tyre size and the new tyre size using the common format width / profile / rim, such as 205/55 R16. The calculator converts each size into dimensions and compares the percentage change.
Original tyre
New tyre
| Measurement | Original | New | Difference |
|---|
Speedometer check
Tyre label decoder
Example: 205/55 R16 means the tyre is 205 mm wide, the sidewall is 55% of the width, and it fits a 16 inch rim. The calculator uses these three values to estimate overall tyre diameter and rolling circumference.
How it works
A tyre size such as 205/55 R16 contains three main dimensions: tread width in millimetres, sidewall profile as a percentage of the width, and rim diameter in inches. The calculator converts the rim size into millimetres, calculates the sidewall height, then adds the two sidewalls to the rim diameter to estimate total tyre diameter.
The rolling circumference is estimated from the diameter. Comparing the old and new circumference tells you how much the speedometer and odometer may be affected. A larger tyre travels further per wheel revolution, so the real speed may be higher than the indicated speed. A smaller tyre travels less distance per revolution, so the real speed may be lower.
Formula used
rim diameter in mm = rim inches × 25.4
total diameter = rim diameter + 2 × sidewall height
circumference = π × total diameter
speed change % = (new circumference ÷ old circumference − 1) × 100
The output is an estimate. Real rolling radius can vary slightly depending on tyre brand, tread depth, pressure, load, wear, speed and construction.
Worked example
Example: 205/55 R16 to 225/45 R17
Original diameter = 16 × 25.4 + 2 × 112.75 = 631.9 mm
New sidewall = 225 × 45 ÷ 100 = 101.25 mm
New diameter = 17 × 25.4 + 2 × 101.25 = 634.3 mm
Diameter change = 634.3 − 631.9 = 2.4 mm
Percentage change ≈ 0.38%
In this example the new tyre is only slightly larger, so the speedometer difference is small.
Common mistakes
Only checking rim size
A tyre can fit the same rim but still have a very different rolling diameter because width and profile also matter.
Ignoring load and speed rating
This calculator compares size only. Your replacement tyre must also meet the correct load index, speed rating and vehicle requirements.
Assuming all brands measure the same
Two tyres with the same labelled size can vary slightly in real-world dimensions due to construction and tread design.
Forgetting clearance
Wider or taller tyres can rub suspension, arches or liners, especially under load or when steering at full lock.
Common questions
What tyre size difference is acceptable?⌄
Will changing tyre size affect my speedometer?⌄
Does this check tyre legality?⌄
What does 225/45 R17 mean?⌄
Sources, assumptions and disclaimer
This calculator is for general comparison only. It estimates tyre geometry from labelled tyre size and does not guarantee physical fitment, road legality, speedometer compliance, ABS/ESP compatibility, insurance acceptance or manufacturer approval.
- Geometry assumption: calculations use the labelled tyre width, aspect ratio and rim diameter. Real measured dimensions can vary by manufacturer, tyre model, tread depth, wear, pressure and vehicle load.
- Speedometer assumption: speedometer effect is estimated from rolling circumference only. Actual speedometer calibration may include manufacturer tolerance.
- Legal tread depth: the UK Highway Code says cars, light vans and light trailers must have at least 1.6 mm tread depth across the central three-quarters of the tyre breadth and around the entire circumference.
- Tyre condition: tyres should also be free from certain cuts and defects. Check condition, pressure and tread regularly, especially before long journeys.
- Safety disclaimer: check your vehicle handbook, tyre placard, wheel width, load index, speed rating, insurance position and professional tyre-fitter advice before changing tyre size.