There are gears for going fast and gears for going slow. Would you like to do both? A lot of us do. We'd like to be able to go from doing 70-mph down the highway to crawling a 1/2-mile of trail in an afternoon. Fortunately, there are solutions to be had these days between choosing gear ratios in transmissions, axles, and transfer cases. If you're building a Jeep drivetrain from scratch or doing some long-term planning, sitting down to design the gear makeup in your drivetrain can make a huge difference in your 4WD performance.
In general, the transmission you use, be it manual or automatic, will have the least effect on your overall gear versatility. Granted, there may be low-speed advantages gained using some granny-gear manual transmissions, and modern overdrive ratios can be great for the highway end of the gearing spectrum. However, you're probably less likely to have as much flexibility in changing transmissions as you do changing axle or transfer case gearing.
For a street-driven Jeep or for any rig that wants to be driven at high speeds, the axle gear ratio will be your next gearing choice after having decided if you'll be using an overdrive transmission. In a high-speed scenario, the transfer case will be in high range with a 1:1 ratio, so axle gears are generally chosen to get the engine humming along in a sweet rpm range when going down the highway.
To figure out your engine speed based on gearing and tire size use:
ENGINE RPM = [SPEED (mph) x FINAL DRIVE RATIO x 336] / TIRE SIZE (inches)
FINAL DRIVE RATIO = [AXLE RATIO x TRANSMISSION GEAR RATIO x TRANSFER CASE RATIO (1:1 in high range)]
Inside the transfer case is where we can find a wide range of choices for overall drivetrain gearing today. Again, the high-range ratio is going to be 1:1, so we're interested in seeing what we can do with low-range gearing. Many early gear-drive transfer cases often used 2:1 low-range gearing. Chain-drive cases with a planetary reduction set are generally about 2.72:1. These ratios typically do well for running mild trails, sand dunes, mud, loose hills, and snow—where you'll wish you had deeper gears is when crawling. Trying to inch along while running large tires will mean slipping the clutch a lot or driving up the fluid temperature in an automatic’s torque convertor. Neither of these conditions is healthy for your drivetrain on a long-term basis.
Today there are replacement low-range gears for a number of gear-drive transfer cases to get the ratio down to between 3:1 and 4:1. An aftermarket gearset replaces the factory gears to change the low-range ratio. An advantage to this solution is that no other external modifications are needed, including driveshafts. Simply install the new gears in the transfer case. One possible downside to this method is the loss of a moderate low-range ratio. If running a manual transmission, you can simply start rolling in a gear higher than first. However, those running automatics may find the transmission shifting very quickly through gears to get the vehicle running at mild trail speeds.
Double Your Gears
In a case where you have a long-wheelbase Jeep, running a "doubler" or two transfer case reductions (sometimes called dual cases) can get you very low gearing. Simply put, this means adding another high-low gear reduction unit between your transmission and some existing transfer case. This can be done by adapting the reduction unit from another transfer case or adding a planetary unit in this location. Occasionally, homebrew solutions will use a second divorced transfer case hooked up behind the main transfer case through a short connecting driveshaft. With dual gear reductions, you end up with four possible transfer case ratios.
For extremely low gearing, aftermarket gears can be combined with a doubler setup. Since adding a doubler moves the transfer case outputs rearward on the vehicle, the front driveshaft length grows while the rear driveshaft length shrinks. Whether or not this is beneficial on any particular Jeep will depend on wheelbase and other drivetrain factors. Sometimes this change improves driveline lengths and angles, and sometimes it can hurt them. In cases where a front driveshaft comes very close to an auto transmission pan, adding a doubler may increase the clearance and allow more uptravel for the front axle or the use of a high-pinion front axle.
Adding a doubler setup usually means also modifying the floorboard and/or console area to accommodate at least one additional shifter. Crossmember modification and a speedometer cable or wire extension are most likely needed. Finally, a small downside to adding a second transfer case is the slight increase in drivetrain play due to another added set of gears.
With all the gearing choices today, you can optimize your Jeep for the engine, tires, and terrain you’re dealing with. You can have good road gearing and still gear down to tackle technical terrain with slow, smooth crawl speed.
Use of an older granny-geared manual transmission will get you a low first-gear ratio, but often an overdrive transmission is desirable for high-speed use.
For a Jeep that sees significant road time, axle gearing is often chosen to restore reasonable gearing after adding larger tires. The main goal is to get a comfortable engine speed at sustained highway speed. Then off-road capability can often be addressed with transfer case gearing.
Deep low-range transfer case gearing is arguably one of the best drivetrain modifications for achieving slow-speed control and traction when crawling. Even small engines can provide great crawling performance with the right low gearing.
Working in the confines of a short-wheelbase rig means adding a doubler could shorten the rear driveshaft too drastically. In such a case, one has to stick with the more compact solution of a single transfer case reduction and run deep gearing in the low range portion.
The NVG241OR RockTrac transfer case was offered in the TJ Wrangler starting in 2003. The aluminum-case, chain-drive unit offers a 4:1 low-range ratio. It's proven to be a stout transfer case and comes from the factory with a fixed yoke output. Aftermarket support allows these transfer cases to be adapted to a number of common manual and auto transmissions. (photo courtesy of JB Conversions)
JB Conversions offers their 4-To-1 LoMax gear set for long- and short-tail Dana 300 transfer cases. Their five-gear set replaces the internal factory cogs to yield a crawl-worthy 4:1 low-range ratio.
The Advance Adapters Rubi-Crawler is a short auxiliary gearbox that fits all ’03-’11 Jeeps using the 42RLE automatic transmission. It's a planetary reduction unit with a 2.72:1 low range that fits between the factory transmission and transfer case (or Advance Adapters Atlas II). It replaces the 42RLE tailhousing with the Rubi-Crawler of same length, meaning no driveshaft modifications are needed. With this you'll have three transfer case speeds, or with a Rubicon model with 4:1 stock range you will have four speeds.
NorthWest Fabworks offers add-on planetary gearboxes. Their Black Box units can be used to turn a two-speed Atlas II into a four-speed transfer case, or can provide an additional 2.72:1 choice to a Dana 300 and other transfer cases.
The Atlas 4-Speed is an all-in-one package with a planetary reduction sitting in front of a gear-drive transfer case. It provides three low-range ratios. The planetary offers a 2.72:1 ratio and the gear-drive low-range ratio can be 1.5:1 to 4.3:1, offering a final low-low ratio of 4.08:1 to 11.7:1. The added planetary unit makes the Atlas 4-Speed about 5 inches longer than the standard Atlas II, by comparison. It's a compact way to gain a doubler package in a short space.
When using very low transfer case gearing behind a manual or automatic transmission, downhill compression braking can be substantial and highly beneficial. This can allow you to keep your foot off the brake pedal and reduce heating up pads and rotors. On the other hand, with such low gearing, your ability to quickly accelerate out of a tricky predicament can be limited.