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Guide · 1 min read

How to read a tire size (225/45R17, decoded)

Width, aspect ratio, wheel size — and why a swap moves your speedo.

SPECS & CONVERSIONS

That string on your sidewall — like 225/45R17 — isn’t random. Three numbers describe the tire, and knowing them keeps you from a speedometer error or a tire that won’t fit.

The three numbers

  • 225 — the tread width in millimeters.
  • 45 — the aspect ratio: the sidewall height as a percentage of the width. Lower means a shorter, sportier sidewall.
  • R17 — radial construction on a 17-inch wheel.

So a 225/45R17 has a 225 mm tread, a sidewall about 45% of that (≈101 mm), on a 17-inch wheel.

225/45 R17 225 —Tread width, in millimetres 45 —Sidewall height, % of width R17 —Radial, on a 17-inch wheel
A tire’s sidewall code, decoded

Why overall diameter matters

Add up the wheel plus two sidewalls and you get the tire’s overall diameter — which sets your gearing and what the speedometer reads. Change it and your speedo drifts. The tire size calculator compares any two sizes and shows the exact error.

The ±3% rule
Keep a new tire’s overall diameter within about 3% of stock. Beyond that you risk speedometer error, rubbing, and ABS/traction quirks.

“Plus sizing” without the side effects

Want a bigger wheel? Go up in wheel diameter while dropping the aspect ratio to keep the overall diameter the same — that’s a “plus size.” The looks change, the speedometer doesn’t. Check any combo against your stock size with the tire size calculator first.

Compare two tire sizes
Tire Size calculator
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