ClarityHow to use this tool
- Enter original tire width, original tire aspect ratio, original tire rim diameter, replacement tire width, replacement tire aspect ratio, replacement tire rim diameter, indicated speed.
- Select Calculate to view the result.
- Check the method and assumptions below before using the result.
Input reference and sample values
Use the units and choices shown for each field. Sample values are provided for practice, not as recommendations.
| Input | Format and units | Sample |
|---|---|---|
| Original tire width | Numeric value. Unit: mm. Minimum: 100. Maximum: 500. | 205 |
| Original tire aspect ratio | Numeric value. Unit: %. Minimum: 10. Maximum: 100. | 55 |
| Original tire rim diameter | Numeric value. Unit: in. Minimum: 8. Maximum: 30. | 16 |
| Replacement tire width | Numeric value. Unit: mm. Minimum: 100. Maximum: 500. | 215 |
| Replacement tire aspect ratio | Numeric value. Unit: %. Minimum: 10. Maximum: 100. | 55 |
| Replacement tire rim diameter | Numeric value. Unit: in. Minimum: 8. Maximum: 30. | 16 |
| Indicated speed | Numeric value. Unit: mph or km/h. Zero or positive. Maximum: 400. | 60 |
Result for these sample inputs
1.74078177 %
- Original diameter (mm)
- 631.9
- Replacement diameter (mm)
- 642.9
- Original circumference (mm)
- 1985.1724
- Replacement circumference (mm)
- 2019.72992
- Theoretical actual speed (same unit)
- 61.0444691
- Ride-height change (mm)
- 5.5
Size comparison only. This does not establish vehicle fitment, load rating, rim compatibility or a safe replacement size.
Select Try the example in the form to load these values and calculate them again. This replaces the current form inputs. Compare the result with the method and worked explanation below.
The method, explained
Sidewall = width × aspect ratio/100. Diameter = 2 × sidewall + rim inches × 25.4 mm. Circumference = π × diameter. Theoretical actual speed = indicated speed × replacement diameter/original diameter.
Using original tire width = 205 mm, original tire aspect ratio = 55 %, original tire rim diameter = 16 in, replacement tire width = 215 mm, replacement tire aspect ratio = 55 %, replacement tire rim diameter = 16 in, indicated speed = 60 mph or km/h, the result is 1.74078177 %. Change these example inputs to match your task; use the method above to check each step.
What to keep in mind
Nominal unloaded dimensions. Actual rolling circumference depends on tire construction, pressure, load and wear. Confirm approved fitment with the vehicle and tire specifications. Planning arithmetic based on your inputs. It does not include live traffic, road restrictions, fares or service availability.
Common questions
How do I read 205/55 R16?
205 is nominal section width in millimeters, 55 means sidewall height is 55% of that width, and 16 is rim diameter in inches. R describes radial construction and is not entered as a number.
Does similar diameter mean the replacement fits?
No. Clearance, wheel width, load and speed ratings, vehicle systems and manufacturer approvals also matter. This calculator compares nominal geometry only.
Methodology maintained by ClarityKit. How these tools are built and checked.