37x12.50R17 — 116.2 inches per revolution

Where it appears

Three vehicles list 37x12.50R17 as a common upgrade: the Jeep Wrangler JL, the Jeep Gladiator JT and the sixth-generation Ford Bronco. Nothing ships it.

The aspect ratio, which flotation codes never state, works out at 80.

The number that actually changes: revolutions per mile

Everything a vehicle knows about speed and distance is counted in wheel revolutions. It has no sense of how big the tire is; it counts pulses and multiplies by a constant fixed when the vehicle was built.

This size turns 545.1 times in a mile. A Wrangler JL's factory 255/75R17 turns 629.1 times. That is 84 fewer revolutions per mile, and every one of the vehicle's distance and speed calculations inherits the difference untouched.

Per turn, the two cover 116.2 in and 100.7 in respectively — a difference of about fifteen and a half inches on each rotation, repeated five hundred times a mile.

What that does over a distance

The diameter change from 255/75R17 is 15.41%. That is the figure that propagates everywhere:

• An indicated 70 mph is an actual 80.8 mph.
• An indicated 100 miles is an actual 115.4 miles.
• A service interval set at 10,000 indicated miles arrives after more than 11,500 real ones.
• Any fuel-economy readout computed from indicated distance reads roughly fifteen per cent low.

None of those are recalculated by anything on the vehicle when the tire changes. A speedometer error is visible if you look for it; the distance errors accumulate quietly and are the ones people notice years later.

From the Bronco's LT315/70R17, which is a much closer starting point at 34.36 in, the same tire is a 7.68% change: 75.4 mph at an indicated 70, and 107.7 miles per hundred indicated.

Ten inches of sidewall

A 17-inch wheel inside a 37-inch tire leaves 10.00 in of sidewall on each side — more than half the tire's height is sidewall, and the wheel accounts for less than half its diameter.

Expressed as an aspect ratio, that is 80. Nothing on the tire says so, and nobody describes a 37 as an 80-series tire, but the shape is closer to a 1980s light-truck size than to anything currently sold in metric notation on a 17-inch wheel. The tallest metric aspect ratio in this catalogue is the 85 on 235/85R16, and after that the numbers drop away quickly.

Why 17 inches, when the tire is this big

It is worth asking why a 37-inch tire is sold on a wheel this small at all, when the rest of the market has been moving the other way. The answer is in the sidewall figure above.

The three vehicles that list this size are the two Jeeps and the Bronco — all short-wheelbase, all with a solid rear axle, all sold on ground clearance rather than on ride refinement. A 17-inch wheel is the smallest any of them is offered with, and it is what leaves the most rubber between the rim and the ground for a given outside diameter. Every inch added to the wheel takes half an inch off each sidewall and gives nothing back in overall height.

The same 37.00 in on a 20-inch wheel leaves 8.50 in of sidewall instead of 10.00 in, and computes to exactly the same 545.1 revolutions per mile.

The 37 that is an inch wider

37x13.50R17 is the same height — 37.00 in exactly, a difference of 0.00 in — on 13.50 in of section width. Circumference, revolutions per mile, speedometer error, odometer drift and every gearing figure are identical between the two. The entire difference is 1.00 in of casing width.

Below and above

35x12.50R17 is 2.00 in shorter at 5.41%, on the same 12.50 in of width. 315/70R17 is 2.64 in shorter and 0.10 in narrower. 34x10.50R17 is 3.00 in shorter and 2.00 in narrower.

Above, 40x13.50R17 stands 40.00 in: +3.00 in, 8.11%, and an indicated 70 mph becomes an actual 75.7 mph. It is the largest size in this catalogue on any wheel.

Where the width goes after 12.50

12.50 in is the width almost every large flotation tire settles at. Step past it and the catalogue thins to three codes: 37x13.50R17, 38x13.50R20 and 40x13.50R17.

Two of those three are on this wheel, and both are heights this size can reach directly — the same 37.00 in an inch wider, or three inches taller and an inch wider at once. The third puts 13.50 in on a 20-inch wheel and 38.00 in overall.

So the widest part of the market is not spread across wheels and heights the way the 12.50 sizes are. It is a short list, and a 17-inch wheel is where most of it lives — which is the same conclusion the sidewall arithmetic reaches from the other direction.