285/70R17 vs 37x12.50R17 — from the factory 33 to a real 37

The Rubicon is unusual among factory vehicles in shipping with an axle ratio chosen for a tire bigger than the one fitted. That head start is exactly what this comparison consumes.

The size of the jump

4.29 in of diameter, from 32.71 in to 37.00 in. As a proportion, 13.12% — larger than every pairing in this set except the two that start on 15-inch wheels.

The sidewall is where all of it lives, since the wheel does not change: 7.85 in becomes 10.00 in. A 10-inch sidewall on a 17-inch wheel is a genuinely extreme proportion, and the derived aspect ratio of 37x12.50R17 is 80 — nothing on the tire says so, and very few people would guess an 80-series from looking at it.

The gearing arithmetic, exactly

A JL Rubicon leaves the factory with 4.10:1 axles. To reproduce the same engine speed at the same road speed on 37s, the arithmetic asks for 4.64:1.

That number does not exist as a product. The ring-and-pinion sets actually manufactured for these axles step 4.56, then 4.70, so the exact answer falls in the gap between two available ratios and the choice becomes which side of it to land on. The nearest is 4.70:1, which slightly overshoots.

For anyone starting from a non-Rubicon 3.73:1, the exact figure is 4.22:1 with 4.27:1 the closest available. The gear ratio calculator will do it for whatever is in your axles now.

None of this is advice about what to fit. It is the conversion of one ratio into another, which is arithmetic and nothing else.

What 13% does to the readings

Every figure in the panel above is 13.12% applied to a different quantity, which is why they all move together and why fixing one fixes none of the others.

The low-speed end is the one worth noticing on a vehicle used off-road: thirteen per cent of a crawling speed is still thirteen per cent, and low-speed control is where a driver's calibration for their own vehicle actually lives. At the other end, an odometer running 13% slow reaches every service interval about three weeks late, permanently.

Width barely moves, which surprises people

11.22 in to 12.50 in — 1.28 in, or 11.40%. That is the same width step as going from the factory 285 to a 35, because 35x12.50R17 and 37x12.50R17 share a section width exactly. Everything gained between a 35 and a 37 is height.

So a Rubicon owner who already clears 35s is not being asked for more room at the steering stop by this change; they are being asked for two more inches of it above the tire and one more inch of ride height at the axle. That is 2.15 in at the axle in total against the factory size.

The count, and what it does to the driveline

616.6 revolutions per mile becomes 545.1 — a reduction of almost twelve per cent in how often the axle comes round. That single figure is the effective gearing change: the engine turns 11.6% fewer times for the same distance covered, in every gear simultaneously, which is why a tire change of this magnitude is discussed in the same breath as a ring-and-pinion swap and a smaller one is not.

Unsprung mass moves in the same direction and is not measurable from a size code. A 37x12.50R17 in an aggressive pattern is a heavy object, and four of them plus a spare is a large addition to a vehicle's rotating and unsprung weight.

The intermediate size this skips

285/70R17 against 35x12.50R17 is the more common single move — 2.29 in and 7.01% — and roughly half of what is described here. Most 37-inch Wranglers arrived there in two stages rather than one, which matters because each stage has its own gearing answer.

What the numbers stop short of

Everything a 37 actually demands: whether it clears at full stuff and full lock, what the spare carrier will hold, and what the axle shafts and brakes are being asked to do with a tire that heavy. Those are questions about a specific vehicle and its parts. The four figures at the top of this page are properties of two size codes, computed exactly, and they end there.