How the calculation works
Actual speed ≈ indicated speed × new tire diameter ÷ stock tire diameter. Percentage difference = (new diameter ÷ stock diameter − 1) × 100. The difference in speed is actual speed minus indicated speed.
Worked example
A change from 30-inch to 32-inch tires has a diameter ratio of 1.0667. At an indicated 50 mph, theoretical actual speed is 53.33 mph: 3.33 mph, or 6.67%, higher.
When the estimate is useful
Use this tool when your speedometer measures rotation and tire diameter has changed. Enter actual mounted tire diameters when possible, measured under comparable conditions. A tire’s sidewall size is nominal; different models with the same printed size can measure differently.
What can change the result
Pressure, load, tire wear, wheel width, sensor location, factory speedometer error, and a previous calibration can affect the comparison. A larger diameter travels farther per revolution and can make a wheel-based speedometer under-read. A smaller tire reverses the relationship.
Frequently asked questions
Does this recalibrate my UTV?
No. It only estimates a change. Follow the manufacturer’s calibration instructions or consult qualified service support.
What if my stock speedometer is already inaccurate?
The output assumes the original calibration was accurate. Any original error carries into the estimate; this tool does not correct it.
Can I enter centimeters and km/h?
Yes. Metric mode converts the current entries and shows km/h results. Ratios do not depend on the unit system.