Speedometer Error Calculator
See how a taller or shorter tire throws off your speedometer and odometer, with the true speed and the corrected readings.
How it works
true speed = indicated × new diameter ÷ stock diameter · error % = (new − stock) ÷ stock × 100 · speedo reading = true speed × stock ÷ new
A speedometer counts how fast the wheels turn and multiplies by the tire’s calibrated circumference to display speed, so it silently assumes the original tire diameter. Fit a taller tire and each revolution covers more ground than the speedo thinks — you are truly going faster than it reads (it reads low), and the odometer under-counts miles. A shorter tire does the reverse. The error is exactly the percentage change in diameter, so a 30-inch tire on a 28-inch calibration reads about 7% slow at every speed, and the odometer clocks 7% fewer miles than you drive.
Worked example
Going from a 28 in to a 30 in tire: the ratio is 30 ÷ 28 = 1.0714, so at an indicated 60 mph you are really doing 60 × 1.0714 ≈ 64.3 mph — the speedo reads about 7.1% slow. Put another way, when you are truly cruising at 60 the speedo shows only 56 mph, and at a true 70 it shows about 65 mph.
Frequently asked questions
Does a bigger tire make the speedometer read high or low?
Low. A taller tire travels farther per revolution than the speedo was calibrated for, so your true speed is higher than the number shown — you could be speeding while the dial looks legal. A smaller tire makes the speedometer read high, showing more speed than you are actually doing.
How does tire size affect the odometer?
The odometer shares the speedometer’s calibration, so it is off by the same percentage. A taller tire under-counts miles (your real mileage is higher than the odometer shows), which can quietly stretch service intervals and inflate a used car’s apparent value; a shorter tire over-counts.
How much tire-size change is acceptable?
Keeping overall diameter within about 3% of stock holds the speedometer error small and avoids upsetting ABS, traction control and transmission shift points that rely on wheel-speed data. Bigger jumps really should be corrected by recalibrating the speedometer or reprogramming the vehicle for the new tire size.
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This calculator is for educational purposes. Double-check important results before acting on them.