VEHICLE COST AND TRIP SCENARIOS
Tire Size Calculator
Compare the overall diameter of an original and alternate metric tire size and estimate speedometer difference.
Review your result
Your result
Ready when you are.
Enter your values and select Calculate. Your result stays on this device.
How it works
Overall diameter = wheel diameter + 2 × (section width × aspect ratio ÷ 100) converted from millimeters to inches.
Assumptions
Uses nominal tire dimensions only. Actual mounted dimensions, load, pressure, rim width, clearance, and manufacturer specifications vary.
About this calculator
This tire size calculator compares nominal overall diameter and the approximate speedometer effect of two common metric tire formats. It does not approve an alternate tire for a vehicle.
How to use it
- Enter the original width, aspect ratio, and wheel diameter.
- Enter the alternate tire dimensions.
- Review overall diameter and percentage difference.
- Confirm approved fitment, ratings, clearance, and calibration before installing a tire.
Use case
tire size calculator: a clear planning check
Compare two width, aspect-ratio, and wheel combinations while clearly separating nominal diameter arithmetic from vehicle fitment, clearance, and safety approval.
Compare nominal tire diameters and the approximate speedometer change between two metric tire sizes.
Source and update
Last reviewed: 2026-08-10
NHTSA — vehicle ownership and safety resourcesFrequently asked questions
How is tire diameter calculated?
Two sidewall heights are added to the wheel diameter after converting millimeters to inches.
Why can tire size affect the speedometer?
A different rolling diameter changes distance traveled per wheel rotation.
Does a similar diameter guarantee fitment?
No. Width, wheel size, offsets, clearance, load and speed ratings, and vehicle approval all matter.
Keep exploring