315/70R17 — 0.10 in narrower than a 35, and 0.64 in shorter

One factory fitment, two upgrade lists, all three off-road

The sixth-generation Ford Bronco is the only vehicle here recorded with 315/70R17 as a factory size, written LT315/70R17. The Jeep Wrangler JL and the Jeep Gladiator JT both list it as a common upgrade, also with the prefix. There is nothing else in the catalogue: no half-ton, no SUV, no mid-size truck.

Where the nickname comes from, and why it survives

A 35x12.50R17 is 35.00 in tall and 12.50 in wide. This size is 0.64 in shorter and 0.10 in narrower.

Ten hundredths of an inch of section width is nothing. It is less than the variation between two manufacturers' interpretations of the same code, and far less than the change a half-inch of extra rim width produces. Stood next to each other, the two tires are the same width. From the front of the vehicle — the angle most people judge a tire from — they look identical.

The height difference is nine times larger and still small: 1.86%. An indicated 70 mph on one is 71.3 mph on the other, and 100 indicated miles becomes 101.9. So the nickname is wrong about diameter, which is the thing the name actually refers to, and approximately right about everything a bystander can see.

It is not halfway between the 33 and the 35

	33x12.50R17	315/70R17	35x12.50R17
Diameter	33.00 in	34.36 in	35.00 in
Section width	12.50 in	12.40 in	12.50 in
Sidewall	8.00 in	8.68 in	9.00 in

It sits 1.36 in above the flotation 33 and 0.64 in below the flotation 35 — roughly two thirds of the way up. And it is 0.10 in narrower than both of them, because both are 12.50 in wide and this one is not.

That is an unusual position to occupy. Most metric sizes that fall between two flotation sizes differ from them substantially in width as well as in height. This one differs in height and barely at all in width, which is precisely why it reads as a 35 rather than as a 34.

The narrow alternative at the same height

35x11.50R17 also stands 35.00 in, on 11.50 in of section width. Against this size that is +0.64 in of height and 0.90 in less width. Set the three of them side by side and the pattern is clear: on a 17-inch wheel, height above 34 in is available at 11.50, 12.40 or 12.50 in of width, and the choice between them changes nothing that any instrument on the vehicle can detect.

Going up

Target	Diameter	Change	Speed at an indicated 70 mph
35x12.50R17	35.00 in	+0.64 in (1.86%)	71.3 mph
37x12.50R17	37.00 in	+2.64 in (7.68%)	75.4 mph
40x13.50R17	40.00 in	+5.64 in (16.41%)	81.5 mph

The bottom row is worth stating plainly: at 16.41%, a speedometer showing 70 mph is describing a vehicle travelling 81.5 mph, and an odometer reading 100 miles has covered 116.4.

Downward, and the other 34

34x10.50R17 is 0.36 in shorter and 1.90 in narrower — the narrowest way to hold roughly this height on this wheel. 33x12.50R17 is 1.36 in shorter at the same 12.50 in width, and 255/80R17 is 1.30 in shorter on a casing 2.36 in narrower.

What it costs on the three vehicles that use it

The Bronco's other 17-inch factory size is LT285/70R17 at 32.71 in, so moving between the two within one model's own option list is +1.65 in and 5.06% — an indicated 70 mph becomes an actual 73.5 mph.

For a Wrangler JL running its 255/75R17, reaching this size is +2.30 in and 7.18%, or 75.0 mph at an indicated 70. It is also +2.36 in of section width, which on that platform is the larger of the two changes.

The same height, bought on bigger wheels

Nothing on an off-road vehicle obliges this height to live on a 17-inch wheel, and the catalogue holds three near-identical alternatives that do not.

• 285/75R18 — 34.83 in, 0.47 in taller, on 8.42 in of sidewall
• 285/65R20 — 34.59 in, 0.22 in taller, on 7.29 in of sidewall
• 275/65R20 — 34.07 in, 0.29 in shorter, on 7.04 in of sidewall

This size carries 8.68 in, more than any of them. All four sit inside three quarters of an inch of each other in overall diameter, so a speedometer cannot tell them apart, and none of the three is ever called a 35 — which suggests the nickname attaches to the wheel and the tread as much as to the number it claims to describe.