TireDiff

Tire size calculator: diameter difference & speedometer error

Enter your current tire size and the size you want to switch to, and see the difference in diameter and the speedometer error.

These are theoretical values based on the size designation.Details
[Disclaimer] These are theoretical values based on the size designation. Actual diameters vary by brand, inflation pressure, load and wear — always verify fitment on the actual vehicle.

Tire A (Current)205/55R16

Tire B (New)225/45R17

205/55R16 : 631.9 mm225/45R17 : 634.3 mmTire A (Current)Tire B (New)
✓

Diameter difference is within the ±3% rule of thumb

Diameter difference +0.38%

Comparison (A → B)

Diameter difference+2.4mm
%+0.38%
Ride Height Change (radius)+1.2mm

Ride height rises by ~1.2 mm

Speedometer Error

+0.38%
Actual speed at an indicated 100 km/h
100.4km/h
205/55R16225/45R17Diameter (mm)631.9634.3Sidewall Height (mm)112.8101.3Circumference (mm)19851993Revs per km504502
205/55R16225/45R17

FAQ / FAQ

How do I calculate tire diameter?

Overall diameter (mm) = rim diameter (inches × 25.4) + sidewall height (width × aspect ratio ÷ 100) × 2. For a 205/55R16 tire: 16×25.4 + 205×0.55×2 ≈ 632 mm.

What happens when tire diameter changes?

A larger diameter makes the speedometer read lower than your actual speed and slightly raises ride height; a smaller one does the opposite. It also affects odometer and fuel-economy readings and increases the risk of rubbing on the fender or suspension.

How much diameter difference is acceptable?

A common rule of thumb is to stay within ±3% of the original size. In Japan, inspection requires the actual speed to be 30.9–42.55 km/h at an indicated 40 km/h (vehicles from 2007), so a large diameter change may fail. Rules vary by country.

How do I choose a size when plus-sizing wheels?

Keep the overall diameter as close to stock as possible. The classic formula per +1 inch of rim: add 10 mm of width and drop the aspect ratio by 5% (e.g. 205/55R16 → 225/45R17). Use this calculator to check the difference stays within ±3%.

Does the actual diameter differ from the calculated value?

This tool computes theoretical values from the size designation. Real diameters vary by several millimeters between brands and tread patterns, and change with inflation pressure, load and wear. Check the manufacturer catalog or measure for exact values.

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Calculation results on this site are theoretical estimates based on your inputs; their accuracy, completeness, and safety are not guaranteed. The site operator assumes no liability for any damages arising from the use of this site.

[Disclaimer] These are theoretical values based on the size designation. Actual diameters vary by brand, inflation pressure, load and wear — always verify fitment on the actual vehicle. Privacy Policy

Diameter difference
+0.38% 100→100.4km/h