Speedometer Error After Plus-Sizing Wheels
Calculate speedometer error when plus-sizing from a 17-inch to an 18-inch wheel setup that changes overall tire diameter from 26 to 26.8 inches.
How to use this tool
- Use the tire diameter calculator to find your original and new tire diameters.
- Enter both diameters and your speedometer reading.
- The result shows your true speed and the percentage speedometer error.
Plus-sizing wheels and tires can slightly change your speedometer accuracy — use this calculator to find your actual speed at highway indicated speeds.
Frequently asked questions
- Does changing tire size affect the speedometer?
- Yes. A larger-diameter tire covers more ground per revolution, so the speedometer reads low — you're actually going faster than it shows. A smaller tire makes it read high.
- How much tire size difference is acceptable?
- Most states allow up to 3% speedometer error. Keep tire diameter within 3% of stock to avoid odometer, ABS, and traction control issues.