Tire Size Converter (P-Metric)

P-metric (215/65R16) → overall diameter, revolutions/mile, sidewall height.

Inputs

Sidewall as % of width. 65 = 65%.

Result

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General calculation reads

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How to use this calculator

  • Enter the three numbers from your tire sidewall.
  • Read overall dia + revs/mile.

About this calculator

P-metric tire codes follow format Width/Aspect R Rim — like 215/65R16. Width is in millimeters; aspect is sidewall height as percentage of width (so 215 × 65% = 140mm sidewall); R = radial; rim diameter is inches. Overall diameter = rim + 2 × sidewall (in inches). Sized changes affect speedometer accuracy (revs/mile changes), gearing (effective ratio), and clearance. Stay within ±3% of stock diameter to avoid speedometer/ABS issues.

Frequently asked

How does this differ from LT/flotation sizes?+
P/passenger metric uses width × aspect × rim. LT (light truck): same format but reinforced. Flotation: 33×12.50R20 — direct OD × width × rim format.
Speedometer error from new tire size?+
Speed reads off wheel rotations × stored circumference. Bigger tires = under-reading speed. ±3% diameter usually within OEM tolerance.
Plus-sizing rules?+
Drop aspect ratio as you raise rim diameter to keep overall dia constant. 215/65R16 (26.0") → 225/55R17 (26.7") → 235/45R18 (26.3").
Why revs/mile matters?+
For odometer accuracy and ABS sensor calibration. Diff > 3% can throw error codes in modern cars.
Square-shoulder vs. round-shoulder tires?+
Affects aspect-ratio-based diameter slightly (~1% off). Calc uses ideal P-metric formula.

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