Guide
How tire sizes work
The code on the sidewall, decoded once: then applied to every size decision.
Two numbering systems
Passenger tires use metric sizing: 205/55R16. Light truck and off-road tires often use flotation sizing: 33X12.50R15. Both describe the same three physical facts, how wide, how tall and which wheel: in different order.
In metric sizing, the width is fixed in millimetres and the height is expressed as a percentage of that width. That is why changing width changes height unless you also change the aspect ratio: a 225/55 is taller than a 205/55 even though both are "55 series".
What actually determines whether a tire fits
- Overall diameter: drives speedometer error, gearing, and clearance to the wheel arch.
- Section width: drives clearance to the strut, inner fender and suspension on full lock.
- Rim diameter: must match the wheel exactly.
- Load index: must meet or exceed the vehicle specification.
- Speed rating: should meet or exceed the specification; lower ratings change handling.
Plus-sizing
Plus-sizing keeps overall diameter roughly constant while increasing wheel diameter and reducing sidewall height: 205/55R16 → 225/45R17 → 245/35R18. Each step adds width and stiffness, which sharpens steering response and reduces ride comfort. The calculator on this site shows exactly how much diameter drift each step introduces, because "roughly constant" is rarely exact.
Where the numbers come from
Dimensions here are calculated from the nominal size code. Real tires vary slightly between manufacturers, and a mounted tire's diameter changes with inflation pressure and tread wear. Use the calculations to narrow the choice and a physical check to confirm it.