Speedometer error is calculated on the tire size comparison, from the two overall diameters. How different are these two tire sizes?

How different are these two tire sizes?

Enter stock and new sizes. See overall diameter, sidewall, Δ%, and speedometer error from public tire geometry.

A size such as 225/65R17 is the section width in millimetres, the aspect ratio, and the rim diameter in inches. Overall diameter (mm) = rim × 25.4 + 2 × sidewall. Sidewall height (mm) = section × aspect / 100.

Actual speed = indicated speed × new overall diameter / stock overall diameter. Percent error = (new diameter − stock diameter) / stock diameter.

Worked example

Stock 225/65R17 has sidewall ১৪৬.২৫ mm and overall diameter ৭২৪.৩ mm. New 235/60R18 has sidewall ১৪১ mm and overall diameter ৭৩৯.২ mm. Diameter change +১৪.৯ mm (+২.০৬ %). At an indicated ৬০ km/h, actual speed is ৬১.২ km/h. Speedometer error +২.০৬ %.

225/65R17 decodes as section ২২৫ mm, aspect ৬৫, rim ১৭ in.

235/60R18 decodes as section ২৩৫ mm, aspect ৬০, rim ১৮ in.

Outlines are drawn at true scale and share a ground line.

Estimates from public tire geometry. Not a fitment certificate and not a legal speedometer certification.

Relative Δ poke

Enter a rim width and an offset (ET) for each wheel. The sketch aligns the hub faces and shows the change in outer lip position. This is relative Δ poke, not fender clearance for a specific car.

Outer position from the hub face (mm) = (rim width in inches × 25.4) / 2 − offset (mm). Relative Δ poke = new − stock. Positive means the new lip sits farther out.

Example wheels, not a car: a 7.5 in rim at ET 45 sits ৫০.২৫ mm from the hub face. A 8.5 in rim at ET 35 sits ৭২.৯৫ mm. Relative Δ poke is +২২.৭ mm.

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