Calculate speedometer error when changing tire sizes. See actual speed vs displayed speed for 60, 80, 100, 120 km/h.
The speedometer error calculator is a specialized tool for precisely analyzing how changes in tire diameter affect a vehicle's speed measurement system. Since vehicle speedometers are programmed based on the revolutions of a specific tire size, installing tires with a different diameter creates a discrepancy between the dashboard reading and actual speed. This tool calculates that difference as an exact percentage, helping drivers avoid speeding fines and better understand their vehicle's true behavior.
The calculation begins by determining the difference in circumference between the original and new tires. Circumference is found by multiplying the diameter by pi (π), and the ratio between the two circumferences directly corresponds to the speed discrepancy. For instance, if a new tire's circumference is 3% larger, it travels 3% further with each revolution, meaning the actual speed is 3% faster than the speedometer indicates. Our calculator applies this linear relationship across various speed ranges (60, 80, 100, 120 km/h) to generate a detailed comparison table.
Enter your original and new tire sizes (width/aspect ratio/rim diameter) to calculate the speedometer error rate. A larger tire diameter means your actual speed is faster than displayed.
An error rate within ±3% is generally acceptable. Larger errors may cause issues with speed limit enforcement and odometer accuracy.
Tire size notation: 205/55R16 → Width 205mm, Aspect Ratio 55%, Rim Diameter 16 inches
The governing formula is error% = (newDiameter − oldDiameter) / oldDiameter × 100. Worked example: replacing a 205/55R16 (632.6 mm overall diameter) with a 225/45R17 (634.3 mm) yields (634.3 − 632.6) / 632.6 × 100 ≈ +0.27%, well inside the safe window. By contrast, jumping to a 245/70R16 (749.3 mm) gives +18.5%, far beyond tolerance.
Most jurisdictions and manufacturers target a ±3% diameter deviation. UNECE Regulation 39 and US FMVSS 127 require speedometers to never read below true speed, and at most read true_speed × 1.10 + 4 km/h, so a speedometer that under-reads after fitting taller tires is the genuinely dangerous direction. A common mistake is comparing only section widths: a 215 and a 225 with different aspect ratios can have identical diameters and zero error, while two tires sharing a width can differ by several percent.
Remember that the error is linear and proportional, so the percentage is identical at every speed; a 3% error means 3 km/h at 100 km/h but only 1.8 km/h at 60 km/h. Manufacturer-stamped diameters are unloaded figures, while the rolling (loaded) radius is roughly 2-3% smaller due to tire deflection, so real-world error can shift slightly from the geometric prediction.