Speedometer Error Calculator
Find out how much your speedometer reads incorrectly after changing tire sizes. Use this when upgrading to larger or smaller tires to understand the true speed discrepancy.
About this calculator
When you change your tire size, the new tires cover a different distance per wheel revolution than the originals. Your speedometer was calibrated for the original tire diameter, so it no longer reads accurately. The error is calculated as: error (%) = ((new_diameter - original_diameter) / original_diameter) × 100. A positive result means your speedometer reads lower than your actual speed (larger tires), while a negative result means it reads higher (smaller tires). For example, going from a 28-inch to a 30-inch tire produces a +7.14% error — your car is going faster than the speedometer shows. Understanding this error matters for both safety and avoiding speeding tickets.
How to use
Suppose your original tire diameter is 28 inches and you are fitting new tires with a diameter of 30 inches. Plug into the formula: error = ((30 - 28) / 28) × 100 = (2 / 28) × 100 = 7.14%. This means your speedometer will read about 7% low — if it shows 60 mph, you are actually traveling roughly 64.3 mph. Enter your specific original and new diameters to see exactly how far off your speedometer will be.
Frequently asked questions
How does changing tire size affect speedometer accuracy?
Your speedometer measures speed by counting wheel rotations per unit of time and multiplying by the tire's circumference. When you install tires with a larger or smaller diameter, the circumference changes but the speedometer's calibration does not. This means the instrument displays a speed based on the old tire size, creating a consistent percentage error. The bigger the difference in diameter, the larger the speedometer error.
What percentage of speedometer error is acceptable after a tire change?
Most automotive experts and legal standards consider errors within ±3% acceptable, since even factory speedometers are typically allowed a small tolerance. An error beyond 5% can become a safety concern, as you may unknowingly exceed speed limits. If your calculation shows an error greater than 3–5%, consider recalibrating your speedometer or choosing a tire size closer to the original diameter. Some vehicles allow recalibration through the onboard computer or a professional shop.
Why does a larger tire diameter make your speedometer read lower than actual speed?
A larger tire covers more ground per revolution than the original tire. Because the speedometer counts revolutions and assumes the original (smaller) circumference, it calculates a shorter distance traveled than is actually the case. The result is that the displayed speed is lower than your true speed — you are moving faster than the gauge indicates. This is why drivers who upsize tires risk unknowingly speeding.