Speedometer Error Calculator
See true speed after a tire size change.
Results update as you type.
How it works
When you fit tyres of a different size, your speedometer keeps counting revolutions as if nothing changed, and the result is a reading that no longer matches reality, which this speedometer error calculator quantifies. Enter your original tyre diameter, the new tyre diameter, and the speed shown on your dash, and it returns your true speed along with the percentage error the change introduced. The reason the error appears is mechanical and unavoidable. A speedometer measures how fast the wheels turn and multiplies by the rolling distance of one revolution, a distance baked in for the factory tyre. A larger tyre covers more ground per turn, so you are actually going faster than the dash shows, while a smaller tyre covers less, so you are going slower than indicated. The instrument never learns about the swap on its own. This matters for more than curiosity. Driving faster than your speedometer suggests can quietly earn speeding tickets, while an undercounting odometer distorts fuel economy figures, service intervals, and resale mileage. Fitting taller off-road tyres or plus-sizing wheels are the usual culprits. By showing both the true speed and the error percentage, the calculator tells you exactly how far off you are and whether the change is small enough to live with or large enough to warrant recalibrating the speedometer through a tuner or dealer.
actual = indicated × new diameter ÷ original diameter
A speedometer counts wheel revolutions and assumes each turn covers the rolling distance of the original tyre. Fit a tyre of a different diameter and each revolution covers a different distance, so the true speed scales by the ratio of the new diameter to the original. Multiplying the indicated speed by that ratio gives the actual speed, and comparing the two as a percentage of the indicated reading gives the speedometer error introduced by the change.
Source:Standard tire circumference and speedometer relationship
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter original tire diameter, new tire diameter and indicated speed.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Actual speed updates instantly as you type — no button to press.
Frequently asked questions
Why does a bigger tire make my speedometer read low?
A larger tyre has a bigger circumference, so it covers more distance with each revolution. Your speedometer counts revolutions and assumes the original tyre's smaller circumference, so it underestimates how far you travel per turn and therefore shows a speed lower than your true speed. The bigger the diameter increase, the larger the gap, which is why lifted trucks with tall tyres often read several miles per hour slow.
Where do I find my tire's actual diameter?
You can measure it directly with a tape from the ground to the top of the tyre, or calculate it from the sidewall size using width, aspect ratio, and wheel diameter. A tyre diameter calculator does this from the printed size. Using the true measured or calculated diameters for both the original and new tyres gives the most accurate speedometer error, since published sizes are nominal approximations.
Is a small speedometer error a problem?
Manufacturers typically build in a slight bias so the speedometer reads a little high, keeping drivers legal, and small errors of a percent or two are generally harmless. Larger errors matter more: reading low can lead to unintended speeding, and any error skews your odometer, affecting recorded mileage, warranty limits, and fuel economy math. If the error exceeds a few percent, consider recalibrating the speedometer.
Does speedometer error affect my odometer too?
Yes. The odometer relies on the same revolution count as the speedometer, so any diameter change distorts recorded distance by the same ratio. Larger tyres make the odometer undercount miles, which can make a vehicle appear to have driven less than it has, while smaller tyres overcount. This affects service intervals, fuel economy calculations, mileage-based warranties, and the mileage shown at resale, so it is worth accounting for.
Can I fix a large speedometer error?
Yes. On many modern vehicles the speedometer can be recalibrated electronically using a programmer, tuner, or dealer service that tells the computer the new tyre size. Some aftermarket modules do the same for older systems. Recalibrating restores accurate speed and distance readings after a significant tyre change. If recalibration is not practical, at least know your error percentage so you can mentally adjust the reading while driving.