Tire Size Calculator
Decode any metric tire size like 225/65R17 and compare it with a planned upgrade. See overall diameter, sidewall height, circumference and revolutions per mile for both sizes, plus the exact speedometer error your new rubber introduces and whether it stays within safe limits.
TheCalculators — Tire Size Calculator
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Speedometer Error
+0.3%
At 60 mph indicated you are actually doing 60.2 mph on 245/55R18
- Current Size
- 225/65R17
- New Size
- 245/55R18
- Old Diameter
- 28.52 in
- New Diameter
- 28.61 in
- Diameter Change
- +0.3%
- Sidewall Old → New
- 146.3 → 134.8 mm
- Revs per Mile
- 707 → 705
- Clearance Change
- +1.2 mm
Tire geometry math
- 1Sidewall heightwidth × aspect ÷ 100 = 225 × 65% = 146.3 mm
- 2Overall diameterwheel + 2 × sidewall = 17 × 25.4 + 2 × 146.3 = 724.3 mm (28.52 in)
- 3Circumferenceπ × diameter = 89.58 in → 707 revs/mile
- 4Speedometer error(C_new ÷ C_old − 1) × 100 = 0.33% → speedo shows less than actual on a taller tire
Side-by-side comparison
Within the commonly accepted ±3% diameter window — speedometer, gearing and electronics will behave essentially as stock. Verify fender clearance with the added width.
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Formula
Diameter = rim + 2 × (width × aspect ÷ 100) · Speedo error % = (C_new ÷ C_old − 1) × 100
Pro tips
- • Keep total diameter within ±3% when plus-sizing — compensate wheel inches with a lower aspect ratio.
- • A wider tire with the same diameter gains grip and cornering stiffness but can cost 1–2 mpg.
FAQ
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