245/65R17 — where crossover fitments start being rated like truck tires

Mid-size SUVs and crossovers use this size — Highlander, Pathfinder, Grand Cherokee, Murano — as do a number of light pickups in base specification.

The boundary on one wheel

Size	Classed as	Diameter	Sidewall
225/65R17	passenger	28.52 in	5.76 in
235/65R17	passenger	29.03 in	6.01 in
245/65R17	crossover	29.54 in	6.27 in
255/65R17	crossover	30.05 in	6.53 in

Four sizes, one wheel, one aspect ratio, and even 0.51 in steps throughout. Somewhere between the second and third rows the market stops treating the size as a car tire and starts treating it as a utility one.

Nothing in the geometry marks that change. It is a fact about what manufacturers build in each size and at what load rating, not about the numbers. Two of these sizes are stocked mainly in passenger constructions and two mainly in crossover and light-truck ones, and the load index tells you which is which — the size code does not.

Same casing width, three wheel diameters

245/60R18 measures 29.57 in and 245/50R20 measures 29.65 in. All three carry the same 9.65 in of section, and the total spread across the three wheels is 0.11 in. This size is the shortest of them and the one with the most sidewall by a clear margin.

What the sidewall does to the published figures

At 6.27 in there is a lot of casing to deflect under load. The 682.8 revolutions per mile above is computed from the free radius of a nominal casing at zero deflection; a manufacturer's measured figure for a real tire in this size, taken loaded and at speed, will be higher. The overall diameter and rolling radius reference sets out why the two differ and which one to use for what.

Four evenly spaced rungs, and none of them a real choice

The table at the top is unusually tidy — one wheel, one aspect ratio, 0.51 in between each rung — and that tidiness hides something. A vehicle delivered on any one of those four was not offered the other three.

225/65R17 sits on compact crossovers, this size on mid-size ones, and 255/65R17 on the light-truck end. The steps between them are small enough that any of the four could physically stand in for its neighbour, and large enough that four different vehicle classes each got their own. From the narrowest to the widest is 1.54 in of diameter and 1.18 in of section width.

That is what an aspect-ratio family looks like when it is built to serve a market rather than to give one buyer a ladder: a smooth progression that nobody actually climbs.