What Does 18×8.5-8 Tire Size Mean? | The Numbers, Decoded

For the full breakdown, see our best 18 X 9.50-8 Tires guide.

Read it as three numbers: 18 is the nominal outside diameter in inches, 8.5 is the nominal section width, and 8 is the rim diameter the tire actually fits. It’s a common size on golf carts, lawn and garden equipment, and small utility trailers across the US.

Breaking Down The Numbers In 18×8.5-8

Each number in 18×8.5-8 pins down one dimension: the height class, the width, and the rim it mounts on. Dealers and parts catalogs often write the same size as 18×8.50-8 or 18/8.50-8 — all three formats mean the same thing, and on a tire chart you might also see 18×8.50R8, where the R simply indicates radial construction.

• 18 — nominal outside diameter, in inches. Nominal means it’s a size class, not a guarantee: the real mounted diameter shifts slightly with rim width, air pressure, and manufacturer tolerances.
• 8.5 — nominal section width, in inches. This is measured at the widest point when the tire is mounted and inflated, and it’s not the same as tread width.
• 8 — rim diameter, in inches. This tire fits 8-inch wheels only. The same numbers ending in -10 fit a different, larger wheel.

Tire charts group these sizes by outside diameter; here’s the 18-inch family on 8-inch rims:

Every size above is an 18-inch outside-diameter class, so the middle figure is the one that changes. If you’re replacing an existing tire, match the size printed on the old sidewall; it’s the fastest way