Tire Size Comparison Calculator
Compare two tire sizes to see diameter difference, speedometer error, and ride height change.
Enter old and new tire sizes in standard format.
Tire size codes encode three critical measurements — section width, aspect ratio, and rim diameter — which together determine the tire’s overall diameter and circumference. Understanding this formula lets you calculate speedometer accuracy, ground clearance, and fitment compatibility.
Tire Designation Format: P225/60R16
- 225 = Section width in millimeters
- 60 = Aspect ratio (sidewall height as % of width)
- 16 = Rim diameter in inches
Sidewall Height:
Sidewall (mm) = Section Width × (Aspect Ratio / 100)
Overall Tire Diameter:
Diameter (inches) = (2 × Sidewall mm / 25.4) + Rim Diameter (inches)
Tire Circumference:
Circumference = π × Diameter
Speedometer Error when changing tire sizes:
Actual Speed = Indicated Speed × (New Circumference / Old Circumference)
Worked Example — P225/60R16:
- Sidewall = 225 × 0.60 = 135 mm
- Sidewall in inches = 135 / 25.4 = 5.315 in
- Diameter = (2 × 5.315) + 16 = 26.63 inches
- Circumference = π × 26.63 = 83.62 inches = 6.97 feet per revolution
Fitting a plus-size tire (P235/55R17):
- Sidewall = 235 × 0.55 = 129.25 mm → 5.09 in
- Diameter = (2 × 5.09) + 17 = 27.18 inches (+0.55 in vs original)
- Speedometer at 60 mph indicated: Actual = 60 × (27.18/26.63) = 61.2 mph
Rule of Thumb: Keep total diameter within ±3% of OEM spec to avoid speedometer inaccuracy, suspension stress, and ABS calibration issues.