Tyre Size Comparison Calculator
Compare your current tyre size with a new size to check diameter change, speedometer error, and fitment impact.
How this tyre equivalent size calculator works
A tyre equivalent size calculator helps you compare two tyre sizes and decide if the replacement is close enough to your original setup. The goal is to keep rolling diameter and circumference as close as possible so your speedometer, odometer, ABS, and traction systems stay accurate.
This tool calculates sidewall height, total diameter, rolling circumference, and revolutions per kilometer for both sizes. It then shows the percentage difference and estimates how much your real speed will differ from the speed shown on your dashboard.
Reading tyre size markings
A tyre size such as 205/55R16 means:
- 205 = section width in millimeters
- 55 = aspect ratio (sidewall is 55% of width)
- R = radial construction
- 16 = wheel diameter in inches
In practical terms, changing any one of these values changes the overall tyre diameter. That can alter gearing feel, ride height, and speed readout.
What counts as an equivalent tyre size?
Typical acceptable range
Most enthusiasts and fitment guides treat a total diameter difference of up to ±3% as a workable maximum. Closer is better. Around ±1% is usually excellent for daily driving.
Why the percentage matters
If the new tyre diameter is larger, your vehicle travels farther per wheel rotation. That means your speedometer may read lower than your true speed. If the new diameter is smaller, the opposite happens: your indicated speed can be higher than your actual speed.
Key outputs explained
- Sidewall height: vertical height of one sidewall.
- Overall diameter: wheel diameter plus two sidewalls.
- Circumference: distance traveled in one full revolution.
- Revolutions per km: how many turns the tyre makes to cover one kilometer.
- Ground clearance change: half of the total diameter difference.
- Speedometer error: estimated real speed compared with indicated speed.
Plus sizing and minus sizing basics
Plus sizing typically means moving to a larger wheel diameter and using a lower-profile tyre so overall diameter stays near stock. For example, 205/55R16 to 225/45R17 is a common comparison.
Minus sizing is the reverse: a smaller wheel with a taller sidewall, often used for winter setups for better comfort and pothole resistance.
Fitment checklist before buying
- Confirm wheel width is suitable for the tyre section width.
- Check load index and speed rating meet or exceed factory requirements.
- Verify inner and outer clearance (strut, suspension, fender, liners).
- Ensure no rubbing at full lock or full compression.
- Keep front/rear rolling diameters matched on AWD systems unless manufacturer allows staggered sizing.
- Review local legal requirements and insurance rules.
Important note about accuracy
This calculator provides geometric estimates based on nominal tyre size. Actual measured dimensions vary by brand, tread design, wheel width, inflation pressure, and tyre wear. Always cross-check with manufacturer data sheets and perform a physical clearance inspection.
Final thoughts
A tyre size comparison tool is one of the easiest ways to avoid expensive fitment mistakes. Use it to keep diameter changes minimal, maintain accurate instrumentation, and protect drivetrain behavior. When in doubt, choose the option with the smallest percentage difference and confirm with a professional tyre shop.