VEHICLE COST AND TRIP SCENARIOS
Compare the overall diameter of an original and alternate metric tire size and estimate speedometer difference.
Your result
Enter your values and select Calculate. Your result stays on this device.
Overall diameter = wheel diameter + 2 × (section width × aspect ratio ÷ 100) converted from millimeters to inches.
Uses nominal tire dimensions only. Actual mounted dimensions, load, pressure, rim width, clearance, and manufacturer specifications vary.
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.
Use case
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.
Build a reliable scenario
Tire Size Calculator helps you check a specific vehicles & travel question with figures that apply to you. Compare the overall diameter of an original and alternate metric tire size and estimate speedometer difference. 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. The page keeps the method and limits visible so the result can be understood, compared, and checked instead of treated as an unexplained answer.
Prepare Original width, Original aspect ratio, Original wheel diameter, Alternate width, Alternate aspect ratio and Alternate wheel diameter. 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. Keep units, dates, currency, and time periods consistent.
The stated method is: Overall diameter = wheel diameter + 2 × (section width × aspect ratio ÷ 100) converted from millimeters to inches. Change one assumption at a time to see which input drives the result, then compare scenarios on the same basis.
Uses nominal tire dimensions only. Actual mounted dimensions, load, pressure, rim width, clearance, and manufacturer specifications vary. A size comparison does not establish vehicle fitment or safety. Confirm approved sizes, load/speed ratings, clearance, and calibration with the vehicle and tire manufacturer or a qualified professional.
Source and update
Last reviewed: 2026-08-10
NHTSA — vehicle ownership and safety resourcesTwo sidewall heights are added to the wheel diameter after converting millimeters to inches.
A different rolling diameter changes distance traveled per wheel rotation.
No. Width, wheel size, offsets, clearance, load and speed ratings, and vehicle approval all matter.
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