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2021-and-newer full-size Bronco • Updated September 30, 2026

The best gear ratio for a Bronco on bigger tires depends on what it started with. Moving a 3.73-geared Bronco from smaller factory tires to 35s is a different change from putting 37s on a Sasquatch-equipped Bronco already running 4.70 gears. Picking a ratio from tire diameter alone skips the information that makes the decision useful.

This Offroad Trading Company guide covers the sixth-generation, full-size Ford Bronco introduced for model year 2021: two-door and four-door builds, manual and automatic transmissions, and the axle checks needed before ordering. Bronco Sport, Bronco II and earlier Broncos use different drivetrains. Raptor owners should read the separate fitment notes below rather than apply non-Raptor parts recommendations.

Start with the Bronco you own

Before shopping, record the model year, engine, transmission, trim, packages, existing axle ratio and differential type at each end. A trim badge alone is not enough: packages and availability change by year, and a previous owner may have swapped an assembly. Build records describe how the vehicle left the factory; service records and physical identification tell you whether it is still configured that way.

Factory ratios seen across this generation include 3.73, 4.27, 4.46 and 4.70, depending on the configuration and year. Treat those as identification possibilities, not a promise that every trim offered all four. Check the original window sticker, the year-specific Ford axle-code information and the installed assembly's identification. Have a qualified drivetrain professional resolve conflicting information before parts are ordered.

Sasquatch changes the starting point

A factory Sasquatch setup commonly pairs nominal 35-inch tires with 4.70 gearing and locking differentials. That is why “you must regear a Bronco for 35s” is too broad. A Bronco already built around that combination has not suffered the same diameter increase as one moving from a smaller tire package. Verify the particular year and vehicle rather than assuming that aftermarket flares, wheels or a Sasquatch-style stance establish its equipment.

Likewise, Badlands does not describe one universal transmission-and-tire combination. Record the hardware, not just the badge. The 2.3-liter manual, 2.3-liter automatic and 2.7-liter automatic can present different launch, shifting and climbing behavior even when tire diameter matches.

Check the tire information properly

Write down the complete tire size, model, wheel size and wheel/tire weight. “37s” is a useful shopping category, but an imprecise calculation input. Use comparable manufacturer diameter or revolutions-per-mile data for the original and replacement tires. Inflation, load and tread wear influence the rolling result. A loaded hub-to-ground measurement doubled is not the same as rolling circumference.

Separate diameter from weight. Diameter changes leverage; extra rotating mass changes acceleration demands. Armor, cargo and a roof load add another demand without changing the ratio formula. A diameter-only calculation therefore restores a geometric relationship, not every aspect of stock performance.

M190, M210, M220: why the axle matters as much as the ratio

The modern Bronco has independent front suspension and a solid rear axle. It is not the same parts application as an older solid-front-axle Bronco or a Jeep simply because a listing contains “Dana 44.” Gear rotation, housing design, bearings, carrier compatibility and installation hardware must match.

Ford Performance's M190-to-M210 upgrade guidance calls for module configuration. An assembly conversion therefore needs a complete mechanical and electronic plan: compatible half-shafts and connections, differential operation, matching rear gearing and the applicable configuration procedure. It is not simply a housing swap because the new unit physically fits beneath the vehicle.

Front and rear must form an approved matching ratio combination. Do not mix 4.88 at one end with 4.70 at the other. Four-wheel drive couples the driveline paths, and incompatible ratios can create driveline stress. A shop should verify the intended matched pair, including any legitimate manufacturer-specific nominal-ratio conventions; “close enough” is not a selection method.

Calculate what bigger tires change

Two calculations answer different questions. For comparable tire diameter inputs:

Using an illustrative 35-to-37-inch change with 4.70 gears, the effective ratio becomes 4.70 × 35 ÷ 37 = 4.45. The axle still physically contains 4.70 gears; 4.45 expresses the tire-size effect relative to the starting tire. Restoring that original relationship requires a calculated 4.70 × 37 ÷ 35 = 4.97.

There is no instruction here to purchase “4.97 gears.” That number is a baseline to compare with actual compatible sets. In this example, 4.88 falls slightly below the calculated baseline and 5.13 slightly above it. Choose after considering load, transmission behavior, cruising RPM and the complete parts application—not by automatically rounding upward.

Rounded calculations using hypothetical measured diameters, not exact factory tire specifications or guaranteed available ratios. They do not establish tire clearance, axle strength or towing capacity.

How to choose Bronco gears for 33-, 35- and 37-inch tires

33-inch tires: assess the change before buying gears

A modest diameter increase on a lightly loaded daily driver may not justify opening both differentials. Establish the actual starting ratio and test the vehicle under its normal load. Does it launch smoothly, climb familiar roads sensibly and offer the low-speed control you need? If it does, keep the gearing budget available for a more pressing need.

Do not blame every downshift on axle ratio. Wind, hills and added aerodynamic drag can require a lower transmission gear even with a well-matched axle setup. New warning lights, harsh shifts, slipping or unusual noise require diagnosis rather than a gearing purchase.

35-inch tires: compare your baseline with a factory-style combination

For a Bronco already running 4.70 gears and a comparable tire diameter, a new tread pattern alone is not a reason to regear. A heavier tire can still change the feel, but the diameter math may be nearly unchanged. Owners starting at 3.73 should make a different comparison: the loss relative to smaller original tires is more substantial.

A 4.46-to-4.70 discussion makes sense only in context. Using the 33-to-35 example above, 4.73 is the calculated baseline, so a compatible 4.70 combination is close geometrically. That does not prove it is the best answer for a loaded expedition build, nor mean that switching from an already suitable 4.46 setup will be worth the installed cost.

37-inch tires: evaluate 4.88 and 5.13, then verify the whole build

For the illustrative 4.70/35 starting point, 4.88 recovers most of the gearing relationship and 5.13 goes somewhat deeper. The difference between the two is about 5.1 percent in shaft speed and ideal torque multiplication at the same transmission gear and tire size. That is meaningful, but it is not a transformation from a highway vehicle into a dedicated crawler.

A driver carrying little extra weight over long highway distances may prefer a different compromise from one carrying armor, water and recovery equipment into steep terrain. Compare the intended loaded use, not an empty test drive. The automatic may select different gears after the change, so calculated top-gear RPM is only part of the decision.

A 37-inch tire also requires a separate review of clearance at full steering lock and suspension compression, wheel offset, CV operating angles, steering loads, brake performance and spare-tire support. Regearing does not create clearance or strengthen every driveline component. A tire that clears while parked may contact the body or suspension when the vehicle articulates.

Beyond 37 inches

At this point the question should be broader than which ratio to order. Define how the Bronco will be used, evaluate axle and steering capacity, and plan the suspension and body clearance together. A numerical ratio appearing in a catalog is not proof that the existing assemblies suit the tire, traction and driving loads.

Highway RPM, the 10-speed automatic and the manual crawler gear

A useful approximate road-speed calculation is:

Engine RPM ≈ mph × axle ratio × transmission ratio × 336 ÷ tire diameter in inches

This assumes a 1:1 high-range transfer case, no clutch or torque-converter slip and a suitable effective tire diameter. Use the transmission ratio for the gear actually engaged. The approximation does not predict shift scheduling, boost, fuel consumption or whether the vehicle can hold that gear on a grade.

The 0.636 and 0.646 examples correspond to published early sixth-generation automatic top-gear and manual top-road-gear ratios respectively. Verify your year and transmission. Values are rounded to the nearest ten RPM and use an illustrative 37-inch diameter.

Why an automatic cannot erase the tire-size change

The 10-speed can select a lower gear to increase total reduction. That flexibility helps, but it does not change the axle ratio. A well-chosen regear may improve how often useful ratios are available under load; it does not guarantee the transmission will remain in tenth. Normal downshifts are part of its operation, and lowest RPM is not automatically the most efficient or comfortable operating point.

The seven-speed manual is six road gears plus a crawler

The crawler gear is a separate low-speed tool, not a seventh highway overdrive. A very low overall crawl ratio can help with controlled movement, but it does not restore normal first-gear launch or passing response after a large tire increase. Consider clutch use, regular road gears and highway behavior as well as crawling.

Mechanical crawl ratio = selected transmission ratio × transfer-case low ratio × axle ratio. For an example using a 6.588 crawler gear, 3.06 low range and 4.70 axles, the result is about 94.75:1. With 5.13 axles and the same other inputs, it becomes about 103.42:1. These are mechanical calculations; they exclude tire diameter and automatic torque-converter behavior.

That exclusion matters: taller tires can increase travel per wheel revolution even when the published crawl ratio stays unchanged. A larger numerical crawl ratio is useful context, not a guarantee of better traction or freedom from wheel hop and shock loading.

Shop the regear as a complete, axle-specific job

Start with ring-and-pinion sets, compare the contents of gear packages, and review master installation kits. These categories perform different jobs. A single gear set normally addresses one differential; an install kit does not change the ratio by itself.

Revolution Gear & Axle: individual gear-set examples

Offroad Trading Company lists Revolution's D44-M210-488R 4.88 front set and D44-M220-488 4.88 rear set with modern Bronco applications. These illustrate how a front and rear purchase differs from ordering one universal “Bronco gear.” Have the installer confirm the exact housing, carrier, locker compatibility and installation requirements for both parts.

Do not carry over all Jeep fitment simply because a gear listing also names JL or JT. Some gear applications overlap; complete axle assemblies, covers, shafts and installation kits are not thereby interchangeable. Also confirm whether ring-gear bolts, bearings, seals and other required parts are included rather than assuming the product photo is a complete two-axle package.

Bronco-specific installation kits

The Revolution 35-M210-Bronco front master install kit and 35-M220-Bronco rear master install kit are separate applications. Their listings include pinion and carrier bearings, shims and other setup components, but identify axle seals as not included. That is precisely the kind of detail to resolve in the quote before the Bronco is disassembled.

Ask the installer to compare a complete package with individually sourced sets and kits. Confirm all part numbers on one written plan. If a particular ratio or brand cannot be verified for your configuration, do not substitute a part based on the name alone.

Protecting the rear differential: Motobilt and Ford Performance

The Motobilt MB4073-B Bronco M220 rear cover is an example of fabricated steel protection. Its listing specifies a 3/8-inch mounting ring, 1/4-inch body and bare-steel finish requiring coating. A stronger cover can help protect the housing opening from trail contact; it does not strengthen the ring-and-pinion teeth or correct an unsuitable ratio.

The Ford Performance M220 heavy-duty rear cover offers another construction option. The current store listing names 2021–2025 Bronco applications, so confirm later-year fitment before purchase. For either cover, check clearance through suspension travel and follow the correct oil-level procedure. A differently positioned fill opening is not permission to overfill the axle.

For the rest of the vehicle, browse Ford Bronco parts and accessories around the same documented build plan. Gear-ratio selection should remain tied to the exact tire, axle and intended load, rather than becoming an excuse to replace parts that already serve the job.

Installation, calibration and break-in belong in the quote

A ring-and-pinion installation is precision differential work. Pinion depth, bearing preload, backlash and tooth-contact pattern interact. Reusing the old shim arrangement without measuring is not a setup method. Ask the installer what measurements are recorded, which bearings and seals will be renewed, and how locker operation and leaks will be checked afterward.

On a Bronco, the estimate should account for front-drive-unit access as well as rear-axle work. Compare the installed job rather than the advertised price of two gear sets. Labor, fluids, required fasteners, calibration and any necessary carrier or assembly changes belong in the same quote. Prices vary too much by hardware and location for one unsupported national total to be useful.

Calibration is a separate task from mechanical gearing

Confirm how the tire-size and axle-ratio changes will be represented in the vehicle's configuration, using a procedure supported for that model year and hardware. Tire-size correction affects indicated road speed and related calculations; software cannot create the torque multiplication of new gears. Conversely, installing gears does not automatically finish the electronic setup.

Afterward, verify road-speed accuracy, normal shifting or clutch behavior, four-wheel-drive operation and locker function through the appropriate service checks. Warning lights, binding, leaks or persistent new gear noise deserve prompt investigation. Do not assume a mechanical problem will disappear during break-in.

Plan the first miles before scheduling a trail trip

Get written break-in and lubricant instructions for the exact gears and differentials installed. They may specify staged driving, cooling intervals, limits on towing or hard use, and an early fluid change. Follow the applicable manufacturer's procedure rather than applying a generic mileage number to every brand. Arrange the first inspection and oil service before you leave the installer.

Regearing may improve loaded drivability, but it does not increase the Bronco's published payload, axle or towing limits. New gearing also does not guarantee better fuel economy: tire weight, tread, aerodynamic changes, speed, terrain and transmission behavior all influence the result.

A practical Bronco regear worksheet

1. Identify: model year, engine, transmission, transfer case, current ratio and both installed axle assemblies.
2. Describe the tires: original and proposed size, comparable diameter or rolling data, wheel offset and wheel/tire weight.
3. Define the use: normal highway speed, terrain, travel load, towing within ratings and expected future modifications.
4. Calculate: effective ratio, restoration baseline and RPM in relevant transmission gears.
5. Confirm hardware: matched front/rear gear sets, carrier and locker compatibility, bearings, seals and every kit exclusion.
6. Plan the complete job: installed quote, supported calibration, inspection, break-in and follow-up service.

Bring that worksheet to a qualified drivetrain installer and use it when selecting parts from Offroad Trading Company. We serve Jeep, Bronco, truck and other off-road owners nationwide with parts and accessories; the vehicle-specific installation and setup should be performed by a qualified professional.

2021+ Bronco gear-ratio questions and answers

Does this guide apply to a 2020 Bronco?

The modern full-size Bronco returned for model year 2021. This guide covers that sixth-generation platform. Verify the model year from the vehicle records; a purchase, announcement or build date is not necessarily the model year.

Can I use this guide to order Bronco Sport gears?

No. Bronco Sport is a different platform. The full-size Bronco axle and gear-set examples here do not establish Bronco Sport fitment. The same applies to Bronco II and earlier generations.

How do I find my Bronco's current axle ratio?

Start with the window sticker, year-specific Ford axle-code information and build records. Then verify the installed assemblies and service history. If an axle has been changed, factory records alone cannot confirm its current ratio.

Do I need to regear a Sasquatch Bronco for 35-inch tires?

Not simply because the tire is called a 35. A factory setup already using comparable tires and 4.70 gears has a different baseline from a smaller-tire Bronco. Compare actual dimensions and evaluate loaded drivability before spending on a regear.

Can I keep 4.70 gears with 37-inch tires?

Possibly, if the verified combination meets your needs and the rest of the vehicle supports the tires. Moving from a 35-inch example to 37 inches makes 4.70 behave like about 4.45 relative to the starting tire. Decide whether that change warrants a regear under your real load and use.

Is 4.88 or 5.13 better for a Bronco on 37s?

Neither wins automatically. At the same tire size and transmission gear, 5.13 provides about 5.1 percent more ideal torque multiplication and engine RPM than 4.88. Compare that trade-off with your starting setup, hills, cargo, transmission behavior and available matched hardware.

Does the 2.7-liter engine mean I can ignore gearing?

No. More engine torque may make the tire change less noticeable, but it does not alter the diameter relationship or identify the axle hardware. Build around the engine, transmission and actual load together.

Does the manual crawler gear eliminate the need to regear?

It can provide strong low-speed reduction, but it does not correct taller gearing in the regular road gears. Evaluate starting on inclines, clutch use and loaded road performance as well as the lowest available crawl ratio.

Can I change only the rear gears?

Not as a complete four-wheel-drive regear when that leaves incompatible front and rear ratios. Plan an approved matched combination. Discuss transport and commissioning with the installer rather than operating the vehicle with an unfinished, mismatched driveline.

Can an M210 gear set fit my M190 front differential?

Do not order it on that assumption. They are different assemblies. Have the supplier and installer verify an M190-specific option or plan a complete compatible assembly conversion with the required mechanical and electronic work.

Will these non-Raptor gear and cover examples fit a Bronco Raptor?

Do not assume so. Raptor uses different axle hardware, and the M210-front/M220-rear examples are for verified non-Raptor applications. A shared Bronco name, tire diameter or nominal ratio is not enough.

Can a programmer replace axle gears?

No. Configuration or tuning can change calculations and control behavior, but it cannot change the mechanical ratio between the pinion and ring gear. Verify the supported calibration procedure separately from the hardware decision.

Will regearing improve fuel economy?

It might improve the operating conditions in a particular build, but there is no guaranteed MPG gain. Tire mass, tread, wind resistance, speed, load and the gears selected by the transmission all matter. Choose for suitable drivability rather than a promised fuel saving.

Does a regear increase the Bronco's tow rating?

No. Do not use an aftermarket ratio change to claim a higher published vehicle rating. Stay within the vehicle, axle, tire and hitch limits for the exact configuration, even if acceleration with a trailer feels better.

Will deeper gears make 37-inch tires fit?

No. Check clearance throughout steering and suspension movement independently. Lift height, wheel offset, body clearance, bump travel, CV angles, steering loads and spare support require their own review.

What should a complete regear quote include?

Both approved gear sets, application-specific installation parts, labor, fluids, necessary carrier or assembly changes, configuration work and post-install checks. Ask about kit exclusions, break-in instructions and the first service. Compare complete written quotes rather than gear-set prices alone.

Should new gears whine during break-in?

Do not dismiss persistent new noise as normal. Report it to the installer, along with any leakage, vibration, warning lights or unusual operation. Break-in is not a repair for an incorrect contact pattern or another installation problem.

When is keeping the factory gears the sensible choice?

When the tire change is modest, the vehicle performs appropriately under its normal load, and there is no verified need that a ratio change would solve. Establish the baseline first; spending more is not automatically a better build.

Choose around your starting point. The same 37-inch tire does not create the same problem on every Bronco. A verified axle combination, realistic tire data and a complete installation plan are more valuable than a ratio chosen because it appears most often in a forum thread.

Technical basis: year-specific Ford Bronco specifications and Ford Performance application guidance, with product examples checked against Offroad Trading Company listings. Calculations are illustrative and do not establish fitment. Check current manufacturer instructions for your exact vehicle and parts.

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