Speedometer Error Calculator
Estimate actual road speed after changing tire diameter and quantify speedometer error
Actual speed
How we calculated it
About the Speedometer Error Calculator
Changing tire size can alter the distance travelled per wheel revolution, causing the speedometer to read differently from actual road speed when the vehicle’s calibration is unchanged. This calculator estimates that difference from original and new tire diameters or circumferences.
A larger tire usually makes actual speed higher than indicated, while a smaller tire usually makes actual speed lower than indicated under a simple wheel-based calibration model.
How to Use the Speedometer Error Calculator
- Enter the original tire size or rolling diameter.
- Enter the new tire size or diameter.
- Enter an indicated speed if you want the corresponding actual speed.
- Calculate percentage error and confirm the sign before interpreting whether the speedometer reads high or low.
How the Calculation Works
For a calibration based on wheel revolutions, actual speed ≈ indicated speed × new circumference ÷ original circumference.
Percentage difference can be expressed as (new circumference/original circumference −1)×100. Since circumference is proportional to diameter, diameter ratio can be used when both tires are modeled geometrically.
Worked example
Speedometer Error Calculator Worked Example
Original diameter 28 in, new diameter 29.4 in at indicated 60 mph
Ratio = 29.4÷28 = 1.05. Actual speed ≈ 60×1.05 = 63 mph. The new tire is about 5% larger in diameter, so the vehicle travels about 5% farther per revolution.
Result guide
Understanding Your Speedometer Error Calculator Result
The result is a geometric calibration estimate. Real tire rolling circumference changes with load, pressure and tread, and some modern vehicles can derive speed from more than one sensor source.
Before you rely on the answer
Assumptions and Limitations
- Original calibration was correct for the original tire.
- Diameter or circumference values represent effective rolling size.
- Vehicle electronics do not automatically recalibrate for the new tire.
Practical tip
Using This Result in Practice
After a significant tire-size change, verify actual speed with an independent reliable reference and recalibrate the vehicle if supported. Do not rely on a rough percentage when legal speed compliance matters.
Related calculations
Useful Calculations to Continue With
If you need to compare the dimensions of two tire codes first, use the Tire Size Calculator.
If you want the change in wheel RPM at a fixed road speed, use the Wheel Speed Calculator.
Common questions
Speedometer Error Calculator FAQ
Do larger tires make the speedometer read low?
Typically yes in a simple wheel-revolution calibration because the vehicle travels farther per revolution than expected.
Can speedometer error be exactly predicted from tire label size?
It is an estimate; real rolling diameter can differ from nominal dimensions.
Why is percentage error undefined at zero indicated speed in some formulas?
A percentage relative to zero would require division by zero, even though both actual and indicated speed are zero.