Speedometer Error After Tire Change Calculator

How it is calculated

Each tire size is entered as section width (mm), aspect ratio (%) and rim diameter (inches), as in 205/55 R16. Overall diameter is D = rim × 25.4 + 2 × width × aspect ÷ 100 in mm: the rim in millimetres plus two sidewalls. The speedometer keeps counting wheel turns as if the old tire were fitted, so true speed = indicated × Dnew ÷ Dold and error % = (true − indicated) ÷ indicated × 100. A positive error means you travel faster than displayed. Speed can be in km/h or mph, since only the ratio matters and the result is in the same unit. All seven values must be above zero, and each tire size must look like a real marking: width 50–1000 mm, aspect ratio 10–100 % and rim 4–60 inches. Anything outside those ranges shows no result. The diameters are for an unloaded tire; real tires flatten slightly under load and wear, so measured values differ by a small amount.

Common mistakes

• Entering the rim size in millimetres or the width in inches. Width is millimetres and rim is inches, as printed on the sidewall; a width of 45 or a rim of 61 is outside the accepted range and gives no result.
• Swapping the width and aspect ratio fields. In 205/55 R16, 205 is the width and 55 is the percentage.
• Assuming a bigger wheel always means a bigger error. The sidewall gets lower with the bigger rim, so a 225/45 R17 can stay within half a percent of a 205/55 R16, while a wider 255/55 R17 is 12.72% off.
• Applying the speed error to the odometer in the wrong direction. If the true speed is higher than indicated, the odometer also under-reads the distance covered.
• Reading the new tire off a catalogue as a loaded size. The result is for nominal diameters, not a measurement from your own car.