How to Read a Tire Size: The Complete Guide to 225/45R18

Look at the sidewall of any tire and you'll see something like 225/45R18. Most people know their tire size, but few actually know what each number means and why it matters. Here's a complete breakdown.

225 — Section Width (mm)

This is how wide the tire is from sidewall to sidewall, measured in millimeters. A 225 tire is 225mm wide. The wider the tire, the more contact patch on the road — which means more grip but also more rolling resistance and potentially more rubbing if you're not careful with fitment.

For most passenger cars, widths range from 195mm to 275mm. Performance builds often go wider: 265, 275, even 285mm on aggressive setups.

45 — Aspect Ratio (%)

This is the sidewall height as a percentage of the section width. A 45 aspect ratio means the sidewall height is 45% of 225mm = 101.25mm tall. Lower aspect ratio = shorter sidewall = more responsive handling but a harsher ride. Higher aspect ratio = taller sidewall = more cushion but less precise feel.

Most performance and stance builds run 35–45 aspect ratios. Daily drivers and SUVs often run 50–65 for ride comfort.

R — Radial Construction

The R means it's a radial tire — the most common construction type on road cars. You'll almost never see anything else on a modern passenger vehicle.

18 — Rim Diameter (inches)

This is the diameter of the wheel the tire is designed to mount on, in inches. An R18 tire goes on an 18-inch wheel. Simple. Make sure your tire's rim diameter matches your wheel size exactly — there's no wiggle room here.

Why This Matters for Fitment

When you change wheel size (say, from 17" to 18"), you need to adjust your tire's aspect ratio to keep the overall diameter the same — otherwise your speedometer will be off and the tire may not clear your fenders or suspension. This is called a plus-size fitment.

Use our Tire Size Comparison Tool to see exactly how two tire sizes compare — overall diameter, sidewall height, circumference, and speedometer error. And use the Fitment Guide to see the right tire sizes for your specific car and wheel size.

Questions about what tires to run on your setup? Come see us at Project JDM, 77-23 Queens Blvd, Elmhurst NY. We carry Ironman iMOVE, Zeta, Headway, Constancy, and other quality tire brands with same-day mounting and balancing available.