Tyre Size Calculator
Compare old and new tyre sizes, overall diameter, sidewall, circumference, ground-clearance change, and speedometer difference.
Use the three numbers printed on a tyre—such as 195/65 R15—to compare a proposed replacement with the original. The calculator explains dimensional changes but cannot confirm wheel, suspension, load, or legal compatibility.
Tyre size comparison
Your calculated results are shown below.
Main Result
+2.82%
Within ±3% comparison band
634.5 mm
Original diameter
652.4 mm
New diameter
+8.9 mm
Ground-clearance change
102.8 km/h
Actual speed at indicated 100
126.8 mm
Original sidewall
123.0 mm
New sidewall
488
New revolutions per km
Tyre Size Calculator
The ±3% band is only a preliminary geometry check, not fitment approval. Confirm approved size, rim width and offset, clearance, load index, speed rating, and pressure with the manufacturer or a qualified tyre professional.
Tyre diameter comparison
Diameter difference shown to two decimalsA commonly used ±3% diameter band is displayed only as an initial screening guide. Always follow the vehicle and tyre manufacturer's approved specifications.
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How the Tyre Size Calculator Works
Enter the original tyre
Add its width, aspect ratio, and rim diameter.
Enter the proposed tyre
Use the exact replacement size printed by the manufacturer.
Compare dimensions
Review diameter, sidewall, circumference, and revolutions per kilometre.
Check fitment professionally
Confirm rim width, clearance, load index, speed rating, and manufacturer approval.
Tyre size calculation
Diameter (mm) = Rim inches x 25.4 + 2 x Width x Aspect ratio / 100
Circumference is π times diameter. Speedometer impact is estimated from the ratio of new diameter to old diameter, and ground-clearance change is half the diameter change.
Example Calculation
Input: 195/65 R15 compared with 205/60 R16
Output: Old and new diameters are calculated and their percentage difference is shown
Common Uses
- • Tyre upsizing
- • Replacement comparison
- • Speedometer error estimate
- • Ground-clearance estimate
Questions about this calculator
195 is nominal section width in millimetres, 65 is sidewall height as a percentage of width, R indicates radial construction, and 15 is the wheel rim diameter in inches.
No. The calculator uses ±3% only as a common preliminary comparison band. Safe fitment also depends on vehicle approval, rim width, offset, load and speed ratings, suspension clearance, steering clearance, and local rules.
A larger rolling diameter travels farther per revolution, so actual speed may be higher than indicated relative to the original tyre. A smaller diameter generally has the opposite effect. Real results also depend on wear, pressure, load, and calibration.
The theoretical clearance change is half the overall diameter difference. This does not account for tyre deflection, pressure, vehicle load, or suspension geometry.
No. It compares geometry only. Confirm the approved tyre size, wheel width and offset, fender and suspension clearance, load index, speed rating, and recommended pressure with the vehicle or tyre manufacturer.
It can. Weight, width, compound, rolling resistance, aerodynamics, and pressure may affect efficiency, steering, braking, ride, and noise. Diameter alone cannot predict those changes.
How to compare tyre sizes
A tyre code combines width, aspect ratio, construction, and rim diameter. Because the sidewall is a percentage of width, changing width can also change total diameter even when the aspect number becomes smaller. Comparing overall diameter helps estimate speedometer and clearance changes.
What to verify before upsizing
- Sizes approved in the owner's manual or door placard
- Wheel diameter, rim width, and offset
- Load index and speed rating
- Clearance at full steering lock and suspension travel
- Recommended cold tyre pressure and applicable regulations
Understanding the results
Diameter difference affects distance travelled per wheel revolution. Circumference is useful for estimating revolutions per kilometre, while half the diameter change approximates the change in static ground clearance. These are mathematical estimates, not a fitment certificate.
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Written & maintained by Aditya Singh
Aditya Singh is a software engineer and the founder of DevCalc, based in Uttar Pradesh, India. He builds and maintains every tool on this site with careful attention to accuracy and user experience. Read more about the author →