Tire Size and Speed Ratio Calculator
Compare nominal tire diameters, circumference, rotations per distance and the ideal speedometer ratio.
YOUR CALCULATION
Result
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Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Find nominal sidewall height from section width and aspect percentage. Add two sidewalls to rim diameter, then compare diameters. Ideal distance and road speed scale with circumference.
Nominal diameter=25.4×rim inches+2×width mm×aspect/100; new actual speed=indicated speed×new diameter/old diameter
Worked example
205/55R16 has nominal diameter 631.9 mm. 225/45R17 has 634.3 mm, about 0.38% larger. A nominal 100 km/h indication corresponds to about 100.38 km/h under this ideal ratio.
Assumptions and input details
Nominal geometric comparison only. Actual loaded rolling circumference varies with construction, load, wear, pressure and speed. The original speedometer is assumed calibrated to the original nominal size. Ground-clearance change is half the diameter difference with all other geometry fixed. No fitment, rim-width, offset, load/speed rating, clearance, legality, ABS or stability-control compatibility is determined.
References: Bridgestone: tire width and aspect ratio
Unit definitions
Unit selection is independent of language. Each selector contains only units for its physical quantity. Calculations use defined base units internally.
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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