Jeep Wrangler (JK): the 9.46% gap between its factory sizes

Two recorded sizes are the same tire, geometrically

255/75R17 and LT255/75R17 both compute to 32.06 in tall on a 10.04 in section. Their diameters are identical to the hundredth of an inch, their sidewalls are identical, and they turn the same 629.1 times per mile.

What separates them is everything the geometry does not describe: the LT casing is built and rated to a light-truck standard with a much higher reference inflation, so its load index and its behaviour under load are different objects. Nothing on the size pages of this site moves between those two codes, and quite a lot on the tire does. That distinction is worth understanding before substituting one for the other.

The base and the top trim are 2.77 in apart

225/75R16 at 29.29 in is a modest tire by any modern standard, and it was fitted to the base JK on a 16-inch wheel. The 32.06 in size is 9.46% larger — a spread of nearly three inches inside one model range.

That has a practical consequence for anyone reading a fitment thread. "A JK on 35s" describes two completely different modifications: from the 16-inch base size, 35x12.50R17 is a +5.71 in, 19.51% change, while from the 17-inch Rubicon size it is +2.94 in and 9.17%. Both are routinely described in the same three words.

At the larger figure, an indicated 70 mph is really 83.7, and 100 miles of odometer is 119.5 miles travelled. That is not a rounding error; it is a different vehicle's worth of instrument drift.

The 33 that is nearly free

From the 32.06 in size, LT285/70R17 is a +0.65 in change — 2.03%. It is the cheapest step on this platform in every sense: the smallest speedometer consequence, the smallest gearing consequence, and a tire only slightly wider than the original.

33x12.50R15 reaches almost the same height, 33.00 in, on a much smaller wheel. Coming from the 16-inch base size that is +3.71 in; coming from the 17-inch size it is +0.94 in, a 2.94% change that would sit inside most substitution tolerances.

Where the recorded axle ratios sit

Three are recorded: 3.21:1, 3.73:1 and 4.10:1. On the 3.21 — the numerically lowest ratio in the set — a step from the base tire to 35s asks for 3.84:1 to restore engine speed, with 3.91:1 the nearest ratio sold. From the 17-inch size on 3.73 gears, the same tire asks for 4.07:1 and 4.10:1 is available almost exactly.

That second case is the reason 4.10 gears and 35s appear together so persistently in JK build lists: the arithmetic happens to land on a catalogue ratio.

A gap in the record, stated plainly

The factory wheel diameters recorded for the JK include an 18-inch option, and no 18-inch tire size appears among the sizes above. That is a gap in the record rather than a claim that no such fitment existed — the wheel diameter is listed precisely because the wheel is documented. What is missing is the tire that went on it.

Your own door placard settles it for your vehicle, and it is the only document that is specific to it rather than to the model.

The limits are mechanical, not arithmetic

Everything above is geometry. What a JK will actually accommodate depends on its suspension, its fender geometry, its steering and its axles, and those vary across a decade of production and every modification made since. Owners' reports about this platform are abundant and are exactly that — reports of what other people fitted to other vehicles.