Marked size versus measured size
The sidewall calculation is based on the tire label. Real mounted diameter can vary by manufacturer, load, pressure, rim width and tread wear.
Example
For a 225/45R17 tire, sidewall height is 225 x 45% = 101.25 mm. Converted to inches and doubled for both sidewalls, then added to the 17-inch rim, the overall diameter is about 25 inches.
Common mistakes
The aspect ratio is a percentage of tire width, not an absolute height. A 45-series tire gets taller when the width is larger.
Limitations
Check clearance, load rating, speed rating, wheel width and manufacturer specifications before changing tire sizes. Diameter changes can affect speedometer reading and vehicle systems.
Last reviewed: 2026-05-17