Bolt 33s or 35s onto an early Bronco still wearing factory gears and the truck goes soft — it labors off the line, hunts on grades, and crawls poorly because the engine never reaches the part of its curve where the torque lives. Run 4.11 rear and 4.10 front with 33s, and 4.56 with 35s. Gear both axles in the same visit so you pay the precision setup labor once, and budget a $30 speedometer driven gear so the odometer stays honest. The gear ratio reference covers the underlying math; this card is the build.
Order the wrong parts and this job stops before it starts, so start here. Every early Bronco, 1966 through 1977, has a Ford 9-inch rear axle. The Dana in these trucks is the front: a closed-knuckle Dana 30 from 1966 to 1971, then a closed-knuckle Dana 44 from mid-1971 through 1977. Factory ratios were 3.50, 4.11, and 4.57, with the lower two typically paired to the 3-speed and the smaller engines.
That split matters more than it sounds, because the two axles are not always cataloged at identical nominal ratios:
Confirm what is in the truck before you order rather than trusting the year. Fifty years is plenty of time for someone to have changed both ends.
The arithmetic for how a tire change affects your gearing is one line:
effective ratio = current ratio × (old tire diameter ÷ new tire diameter)
The factory tire on these trucks was roughly 30 inches, so:
That gap is why the documented early Bronco baseline, echoed in the Sexton build guide, is 4.56 with 35s and 4.11/4.10 with 33s — restoring the stock ratio plus one meaningful step of extra crawl. A heavy 35 behind a tired small-block, or a truck that spends its life in low range, sometimes wants 4.88. A 31-inch tire on a mostly-street truck is livable on 4.11 and does not require the job at all.
Axle ratio is one of three multipliers, and on an early Bronco it is often the one that matters least:
crawl ratio = transmission first gear × transfer case low range × axle ratio
The Dana 20 transfer case low range is 2.46:1. Then:
Roughly 40:1 is where a truck starts crawling comfortably, and 50:1 or better is where you stop riding the clutch on ledges. The uncomfortable conclusion is that a factory-transmission early Bronco is gear-limited at the transmission, not the axle, and no realistic axle ratio fixes that alone. If your goal is genuine rock crawling rather than restoring drivability, price a granny-low transmission or a low-range kit for the Dana 20 alongside the gears. If your goal is a Bronco that drives properly on 33s or 35s, the axle ratio by itself does that job well.
The Dana 20 is a part-time case, so a front-to-rear ratio mismatch does not hurt anything in 2WD. It destroys the driveline the moment you engage 4WD — which is precisely when you cannot afford it. Gear both ends to the same number.
There is also a straightforward economic argument. The expensive part of this job is not the gears, it is the setup: pinion depth, backlash, and bearing preload, on two axles. Doing both at once means one round of that labor and one period with the truck off the road. And if a locker is anywhere in your plan, install it in the same teardown — the carrier is already out.
The rear has an advantage the front does not. The Ford 9-inch is a drop-out design: the ring, pinion, and carrier all live in a third member that unbolts from the housing. You can pull it, have it set up on a bench, and reinstall — or buy a second third member already built with your gears and locker and swap it in an afternoon, keeping the original intact as a known-good spare. The Dana front is an integral-carrier axle and has to be set up in the housing.
Follow the gear manufacturer's specifications over any general guidance, including this page.
Setup is where gear jobs fail, not parts quality. A pattern that looks close but sits wrong produces a howl at 45 mph and a destroyed gear set somewhere around 8,000 miles. If you have not set up a third member before, the drop-out rear gives you a clean way to buy expertise without buying a bearing press and a case spreader.
Buy a master kit per axle, not loose bearings. New carrier and pinion bearings, races, crush sleeve, shims, and seals. Reusing a bearing to save $60 on a job with $600 of labor in it is a poor trade.
Do not stack this on weak shafts. Lower gears multiply torque into the axle shafts. If the truck still runs 28-spline small-bearing rear shafts and you are adding a locker and 35s, price 31-spline chromoly shafts at the same time.
Recheck the driveshaft angles if the truck has been lifted. Gears do not cause vibration, but a gear job is when people first drive the truck hard enough to notice one that was already there — see the driveshaft and SYE guide.
DIY, both axles, parts only: $560–800. Shop-installed, both axles: $1,400–2,100. Combining a locker install into the same teardown adds the locker's price plus little or no extra labor, which makes it the cheapest time you will ever buy that upgrade. Re-gear whenever you move up in tire size, match the ratios front to rear, and be honest about whether your real goal is drivability — which gears deliver — or crawl ratio, which needs the transmission addressed too.
| Part | Vendor | Est. price |
|---|---|---|
| Ring and pinion, Ford 9-inch rear (4.11 / 4.56 / 4.88) | Yukon Gear / Motive Gear / Richmond | ~$200 |
| Ring and pinion, Dana 30 or Dana 44 front (4.10 / 4.56 / 4.88) | Yukon Gear / Motive Gear | ~$190 |
| Master install kit, per axle (bearings, races, shims, seals, crush sleeve) | Yukon / Timken / Wild Horses 4x4 | ~$130 |
| Dana 20 speedometer driven gear (recalibration) | Wild Horses 4x4 / Bronco Graveyard | ~$30 |
| Gear oil, 80W-90 or 75W-140 GL-5 (both axles) | Valvoline / Lucas / Red Line | ~$55 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.
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