215/75R15 — light-truck geometry on a 15-inch wheel

This is the first rung of the light-truck range on a 15-inch wheel. Below it the 15-inch catalogue is entirely passenger sizes topping out at 205/65R15 and 25.49 in; above it come 225/75R15 and 235/75R15, then the flotation sizes.

The three 75-series fifteens

Size	Diameter	Section	Sidewall
215/75R15	27.70 in	8.46 in	6.35 in
225/75R15	28.29 in	8.86 in	6.64 in
235/75R15	28.88 in	9.25 in	6.94 in

Even steps of 0.59 in in diameter and 0.39 in in section width for every 10 mm. Three sizes, one wheel, one aspect ratio, and a total spread of 1.18 in.

Its closest match is on a 17-inch wheel

225/60R17 measures 27.63 in — 0.07 in below this size, 0.24%. One is a light-truck size on a small steel wheel with 6.35 in of sidewall; the other is a crossover size on a 17 with 5.31 in. They roll identically.

That pairing shows up more often than it sounds like it should, because the vehicles that used 215/75R15 in the 1990s were replaced by vehicles using something very like 225/60R17, and the two generations end up compared.

Trailers and older light trucks

Marked ST215/75R15, this is a widely fitted trailer size. The ST prefix designates a casing built to a trailer standard rather than a passenger or light-truck one, with different construction and a load rating derived at a higher reference inflation. The geometry is identical; nothing else is. The tire construction codes reference sets out what each service prefix means.

On the road-vehicle side the size belonged to compact pickups, small SUVs and full-size vans of an earlier generation, and it survives as a replacement fitment for them.

Three service types, one set of dimensions

Passenger, light-truck and trailer casings all exist under these eight characters, and their rated capacities are nowhere near each other. Nothing in the geometry distinguishes them; the prefix and the index printed after the size do all of the work, which is unusual even by the standards of a notation that hides a lot.