Formula
Each diameter adds both sidewalls to the rim. The percent is the change from the current tire:
percent = (new diameter − current diameter) / current diameter × 100
Sidewall height is section width times the aspect ratio, converted at 25.4 millimeters per inch. The tire size page reports one tire, including its circumference.
Assumptions
- Both sizes are metric passenger markings.
- The diameters are nominal, not measured on the car.
- The speedometer was correct for the current size.
Worked example
Moving from 225/45R17 to 225/50R17 changes the indicated speed by 3.55%.
Input definitions
- Speedometer change
- The percent by which the new diameter differs. Positive means the tire is taller.
- Aspect
- Sidewall height as a percent of the section width.
Limitations
This uses the nominal size, not a mounted measurement. Confirm the load index, speed rating, and clearance before the tire is installed.
Questions
What does a positive percent mean?
The new tire is taller. At a true 60 mph the speedometer, still calibrated to the shorter tire, reads low by about that percent.
Where is a single tire's diameter?
On the tire size page. This page exists to compare two sizes.