How to read tire size numbers
Decode tire size numbers: section width in millimeters, aspect ratio as a percent of width, radial construction, rim inches, plus how to sanity-check overall diameter before buying.
The three-number core
Most passenger tires print something like 225/45R18. The first number is section width in millimeters. The second is the aspect ratio: sidewall height equals that percent of the width. R means radial construction, and the last number is rim bead seat diameter in inches.
Common misunderstandings
- The middle number is not sidewall millimeters—it is a percent of width.
- The printed width is a nominal engineering label; tread width varies slightly by model.
- Rim diameter is not overall tire height—overall height includes two sidewalls plus the rim.
Why this matters before a wheel upgrade
Once you can read the sidewall, you can type the same numbers into a tire comparison tool and compare diameter to a candidate tire. Try a pre-filled example like 205/55R16 vs 225/45R17 to see how small label changes alter rollout.
Frequently asked questions
What does R mean on a tire?
How do I compare two tires fairly?
Try the free tire size calculator
Open the interactive tire size calculator to compare diameters, see a wheel size comparison side by side, and review a speedometer difference table before you buy tires or wheels. It works like a quick tire fitment checker for geometry—not a substitute for a professional install bay.