Ford Bronco 6th gen: a 4.31 in spread between factory sizes

The factory range is the story

Five recorded factory sizes:

Size	Diameter
255/65R17	30.05 in
255/70R16	30.06 in
255/75R17	32.06 in
LT285/70R17	32.71 in
LT315/70R17	34.36 in

The two smallest are within 0.01 in of each other on different wheels — a 16-inch and a 17-inch fitment holding the same rolling diameter, which is a deliberate calibration decision rather than a coincidence.

The largest is the interesting one. LT315/70R17 at 34.36 in is the factory 35-class fitment that arrives with the off-road package, and it means the top of this range starts where most owners of other platforms are trying to get to.

Four axle ratios, and they are not decoration

3.73:1, 4.27:1, 4.46:1 and 4.70:1 are all recorded. That spread exists precisely because the tire spread exists: a platform that ships on tires from 30 to 34.4 inches cannot use one final drive.

The arithmetic of the pairing is visible in the restoring figures. Going from 255/70R16 — the smallest recorded tire — to 35x12.50R17 is a 16.45% increase, and on the 3.73 axle the restoring ratio is 4.34:1. The factory's own 4.27 is close to that, and its 4.46 slightly past it. Ford's ratio ladder for this platform is, in effect, a tire-diameter ladder expressed in gears.

What each starting point pays for 35s

35x12.50R17 measures 35.00 in:

• From the factory LT315/70R17: +0.64 in, 1.86%. Barely a change.
• From LT285/70R17: +2.29 in, 7.01%.
• From 255/75R17: +2.94 in, 9.17%.
• From 255/70R16: +4.94 in, 16.45%. An indicated 70 mph is 81.5, and 100 odometer miles is 116.5 travelled.

The last figure is nine times the first. Both are described in the same three words — "put 35s on it" — and both are being said about a Bronco.

The 37-inch question

37x12.50R17 from the factory 35-class size is +2.64 in and 7.68% — a smaller proportional change than a base-model owner makes going to 35s. From the 16-inch base size it is +6.94 in and 23.11%, at which point an indicated 70 mph is 86.2 and the odometer is out by nearly a quarter.

Nothing about those numbers says a 37 fits any Bronco. They say what the geometry does.

The intermediate that gets overlooked

LT295/70R17 at 33.26 in sits between the 33-class and 35-class factory sizes. From LT285/70R17 it is +0.55 in — 1.69% — which is close to a substitution. From 255/75R17 it is +1.20 in, 3.75%.

For an owner who wants a wider, more aggressive tire without moving the speedometer or the gearing, that is the size the arithmetic points at, and it is the one least often discussed.

Going down is also a move

LT295/70R17 measured from the factory 35-class fitment is a negative change: 33.26 in against 34.36 in, or −3.21%. An owner fitting it is reducing rolling diameter, and the speedometer would begin to over-read by that proportion.

That is a real choice on this platform rather than a curiosity. On a vehicle geared for the tallest factory option, a shorter tire raises engine speed at any given road speed rather than lowering it — the opposite of what a size change usually does — and it arrives with less unsprung mass and a wider selection of tire models than the 35-class sizes offer.

It also illustrates why the direction of a change matters as much as its size: every figure on this site is signed, and a smaller tire moves each of them the other way.

The usual boundary

Clearance on this platform is a function of the suspension package, the fender flares, the front bumper and how far the steering turns — and this is a vehicle with removable flares and a bolt-together front end, so two examples of the same trim can differ by the time they reach a tire shop. The 4.31 in the factory itself spans is the honest reminder: the size code tells you the tire's dimensions exactly and tells you nothing about the space around it.