Jeep Gladiator (JT) tire sizes and what the bed changes

The recorded sizes, and why the LT prefix matters more here

Three factory fitments are recorded: 245/75R17 at 31.47 in, 255/70R18 at 32.06 in, and LT285/70R17 at 32.71 in. The top size is an LT-construction 33, and on a vehicle rated to carry a bed load the light-truck casing is doing work it is not doing on a two-door Wrangler.

That is the substantive difference between this platform and its short-wheelbase sibling from a tire arithmetic point of view. The geometry is shared; the load case is not. A tire substitution here has to clear the load index the placard was written against, and the placard is where that figure lives.

The step to 35s, priced from each starting point

35x12.50R17 is the size this platform is most often moved to:

• From LT285/70R17: +2.29 in, 7.01%. An indicated 70 mph is 74.9.
• From 255/70R18: +2.94 in, 9.19%.
• From 245/75R17: +3.53 in, 11.22%, and 100 odometer miles is 111.2 travelled.

The middle case is the one worth noticing. Owners on the 18-inch wheel are making a 9.19% change, which is materially larger than the 7.01% the Rubicon owner beside them at the trailhead is making to the same tire size.

Both recorded ratios, and where they land

Two factory axle ratios are recorded: 3.73:1 and 4.10:1.

From	To	Change	Restoring ratio on 3.73	On 4.10
LT285/70R17	35x12.50R17	+2.29 in	3.99:1	4.39:1
LT285/70R17	LT315/70R17	+1.65 in	3.92:1	4.31:1
LT285/70R17	37x12.50R17	+4.29 in	4.22:1	4.64:1
245/75R17	37x12.50R17	+5.53 in	4.39:1	4.82:1

None of those restoring figures is a ratio you can buy — the nearest catalogue steps are 3.91, 4.30, 4.27, 4.56 and so on. That is normal, and the residual is a percent or two of engine speed either way.

Gearing matters more on a truck that tows

The last row is the interesting one. Going from the base tire to 37s is a 17.58% diameter increase, which reduces torque at the road by the same proportion and asks the transmission to hold lower gears for longer. On a vehicle rated for a tongue weight and a payload, that is a different problem than it is on an unladen short-wheelbase Wrangler making the same change.

The arithmetic is identical; what it costs is not.

The intermediate sizes exist for a reason

LT315/70R17 at 34.36 in sits between the factory 33 and a 35. From the Rubicon's size it is a +1.65 in, 5.06% change, and a needle showing 70 mph corresponds to 73.5 on the road. For an owner who wants more tire without a regear conversation, that is the size the arithmetic points at.

The fifth tire is a Gladiator-specific problem

Unlike the Wrangler, this platform carries its spare underneath the bed rather than on a tailgate carrier. A matching spare is part of the cost of a size change on any full-time four-wheel-drive vehicle — a spare of a different rolling diameter is a circumference mismatch the driveline has to absorb — and here it also has to physically fit the under-bed location.

That constraint arrives at a different tire size than the fender clearance does, and it is easy to discover after the other four are already mounted.

Clearance is a question about your Gladiator

Bed, wheelbase, suspension, bumper and steering geometry all decide what turns without contact, and this page has no access to any of them. What owners of this platform commonly fit is recorded above as exactly that — what owners commonly fit — and the door placard on your own truck is the record of what it was delivered with.