Tyre Size Calculator & Comparison Tool
Enter your current and new tyre dimensions to compare diameter, circumference, revs per distance, and speedometer difference.
Tip: Most experts recommend staying within ±3% overall diameter difference from OEM size.
How to Use This Tyre Size Calculator
A tyre size calculator helps you compare your existing tyres with a proposed replacement size. This is useful when upgrading wheels, searching for better ride comfort, improving grip, or selecting winter tyres. In practical terms, the key question is: how much does the overall rolling diameter change?
If tyre diameter changes too much, your speedometer and odometer can become inaccurate, and your car's handling, gearing, and even clearance can be affected.
Understanding Tyre Size Numbers
A size like 205/55R16 means:
- 205 = tyre width in millimeters
- 55 = aspect ratio (sidewall height is 55% of width)
- R16 = wheel/rim diameter in inches
The calculator takes these values, converts where needed, and computes the key geometry values used in fitment checks.
What the Calculator Computes
1) Sidewall Height
Sidewall height = width × (aspect ratio / 100). A taller sidewall generally improves comfort but can feel less sharp in cornering.
2) Overall Diameter
Overall diameter = rim diameter (in mm) + 2 × sidewall height. This is the most important value for speed and gearing impact.
3) Circumference
Circumference = π × diameter. Circumference determines how far your car travels in one wheel rotation.
4) Revs per Distance
Smaller tyres spin more times per mile/km; larger tyres spin fewer times. This affects odometer readings and effective final drive behavior.
5) Speedometer Difference
If the new tyre is larger, your true speed is usually higher than indicated. If smaller, true speed is lower than indicated.
Why Diameter Difference Matters
- Speedometer accuracy: A large difference can cause legal and safety issues.
- ABS and traction systems: Modern systems expect wheel speeds in a normal range.
- Suspension and clearance: Larger diameters may rub on arches or liners.
- Fuel economy and acceleration: Effective gearing changes with diameter.
Practical Fitment Guidance
For most daily-driven vehicles, keeping the diameter change within ±1% to ±3% is considered sensible. For performance, AWD, and heavily loaded vehicles, tighter tolerances are often preferred. Always verify load index, speed rating, and local regulations before fitting a different size.
Frequently Asked Questions
Can I increase wheel size and keep similar rolling diameter?
Yes. This is commonly called a plus-size setup. You increase rim diameter but reduce sidewall height so total diameter remains close to stock.
Is wider always better?
Not always. Wider tyres can improve dry grip but may increase noise, rolling resistance, and hydroplaning risk in some conditions.
Do I need to change all four tyres?
In most cases, yes—especially for AWD vehicles where mixed diameters can stress drivetrain components.
Final Thoughts
This size tyre calculator is designed as a fast decision aid when evaluating alternative tyre sizes. Use it to shortlist options, then confirm physical clearance, tyre load/speed ratings, wheel offset, and manufacturer guidance before purchase.