Tyre Size & Speedometer Calculator

Compare tyre width, aspect ratio, rim size, overall diameter difference, and speedometer reading errors when upgrading car tires.

Interactive Tyre Size & Speedometer Calculator
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Formula & Calculation Methodology

Tyre Diameter = (Rim Diameter × 25.4) + 2 × (Tyre Width × Aspect Ratio / 100). Speed Variance % = ((New Diameter - Old Diameter) ÷ Old Diameter) × 100.

D = (Rim 	imes 25.4) + 2 	imes left(rac{Width 	imes Ratio}{100}
ight)

Step-by-Step Worked Example

Original Tyre: 185/65 R15 | Upgraded Tyre: 195/60 R15

  1. 1. Original Diameter = (15 × 25.4) + 2 × (185 × 0.65) = 381 + 240.5 = 621.5 mm
  2. 2. New Diameter = (15 × 25.4) + 2 × (195 × 0.60) = 381 + 234.0 = 615.0 mm
  3. 3. Difference = 615.0 - 621.5 = -6.5 mm (-1.05%)
  4. 4. Speedometer Reading @ 100 km/h: Shows 100 km/h, Actual Speed = 98.95 km/h
Result: Diameter Variance: -1.05% (Safe: Within ±2.5% safe limit) | Speedometer Difference: -1.05 km/h @ 100 km/h

Compare Tyre Upgrade Sizes and Speedometer Error

Thinking of upgrading car or SUV tyres? Use our Tyre Size Comparison Calculator to check changes in overall rolling diameter, sidewall height, ground clearance impact, and speedometer reading errors.

Frequently Asked Questions

What is the maximum safe tyre diameter difference?

It is recommended to keep tyre diameter variance within ±2.5% of original factory specs.

Does a wider tyre reduce car mileage?

Yes, wider tyres increase road contact friction (rolling resistance), which can slightly reduce fuel efficiency by 0.5 to 1.5 km/L while improving cornering grip.

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