Speedometer error is calculated on the tire size comparison, from the two overall diameters. 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.
Stock 225/65R17 has sidewall 146,25 mm and overall diameter 724,3 mm. New 235/60R18 has sidewall 141 mm and overall diameter 739,2 mm. Diameter change +14,9 mm (+2,06 %). At an indicated 60 km/h, actual speed is 61,2 km/h. Speedometer error +2,06 %.
225/65R17 decodes as section 225 mm, aspect 65, rim 17 in.
235/60R18 decodes as section 235 mm, aspect 60, rim 18 in.
Outlines are drawn at true scale and share a ground line.
Diameter change
+14,9 mm
Δ%
+2,06 %
Vitesse réelle
61,2 km/h
Erreur du compteur
+2,06 %
| 225/65R17 | 235/60R18 | Δ | |
|---|---|---|---|
| Section width | 225 mm | 235 mm | +10 mm |
| Sidewall height | 146,25 mm | 141 mm | -5,25 mm |
| Diamètre hors tout | 724,3 mm | 739,2 mm | +14,9 mm |
| Circonférence | 2 275,5 mm | 2 322,3 mm | +46,8 mm |
| Tours | 439,5 /km | 430,6 /km | -8,9 /km |
Estimates from public tire geometry. Not a fitment certificate and not a legal speedometer certification.
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 50,25 mm from the hub face. A 8.5 in rim at ET 35 sits 72,95 mm. Relative Δ poke is +22,7 mm.