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Gearing: Trans vs. T-Case for Off-Road

Posted in Features on December 5, 2018
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Photographers: Harry Wagner

Many of us in the off-road world are somewhat obsessed with gearing. Numerous gearing solutions can help you get down the road and trail even after you make major modifications. You can gear axles down to compensate for larger tires, swap to a “granny geared” transmission, search for ultralow gearsets to swap into your transfer case, double up on transfer cases or add range boxes, and build around a transmission (or transfer case) that has overdrive, all in search of a solution. Still, if you screw up your formula you may end up with a vehicle that stalls from a stop, revs too high on the highway, or has one gear (or more) that you never use on-road or off. Fortunately there is more than one way to skin the cat that is gearing. Aftermarket applications abound for lower-than-stock transfer case gearing, range boxes, and/or transmission swaps. What does your favorite magazine writer think works the best?

A granny gear four-speed refers to any number of old-school manual truck transmissions from the SM420, T18, T98, SM465, NP435, and others from the late 1940s to the late 1980s. Most of these feature a First gear with a very deep ratio in the 6:32-7:05 range to allow the truck to crawl slowly and have good torque multiplication for pulling or plowing. The downside is a huge split between the gears, which can hamper driveability on the street.

Christian Hazel

Editor
I’m all about a granny-geared transmission. My all-time favorite manual transmission is the NP435 with a 6.68:1 First gear, followed closely by the Muncie SM420 and SM465 in that order. I’d much rather have a granny First gear than a 4:1 (or lower) ratio in my T-case because I find 4:1 T-case gearing limiting when my wheeling requires any real wheel speed like mud, snotty trails, or sand dunes.

That said, if I absolutely had to choose between a granny tranny and a low gearset in my T-case, I’d take neither. I’d much rather have some form of crawler box like an Offroad Design Doubler or Magnum; a four-speed Atlas, Klune-V, or NorthWest Fab; or any underdrive gearbox that’s independent of my transmission and T-case gearing because it offers the best of all worlds. Need to maintain wheel speed? Put your primary T-case in Low and have at it. Need double-secret probation low, low? Engage your crawler box and get technical. Or how about a granny-geared transmission, a crawler box, and a low-range gearset in your T-case? Man, with that you could crawl slow enough to travel backwards in time, yet still have wheel speed when you want it.

Toyotas, Suzukis, and other imports with low-powered four-cylinder engines almost require dual T-cases to obtain enough low-range gearing to make them survive technical hardcore off-roading. Without sufficient low-range gearing like that provided by this Marlin Crawler dual T-case setup (which stacks a low-range portion of the Toyota T-case in front of another Toyota T-case), you will find your clutch going up in smoke before you’re 100 yards down the trail.

Trent McGee

Nuts & Bolts author
At one time I would have leaned toward transfer case gears, but the advent of things like eight-speed automatics transmissions and four-speed transfer cases has changed the landscape of gearing possibilities substantially. Because of all the choices and options available nowadays, I don’t think it really matters which route you go. You can have a mild transfer case low range and a deep one, so there’s no more compromise between tire speed and crawl ratio. Modern transmissions do an excellent job keeping engines in their optimum powerband, so you have a fairly deep First gear ratio along with one and sometimes two overdrive ratios. Some old-timers may scoff, but this is an instance where modern technology has made a lot of improvements to the off-road world.

Most would argue that unless all you’re doing off-road is low-speed technical rockcrawling then leave the 4:1 T-case gearsets on the shelf. Unless you’re dealing with a very modern vehicle that features an automatic with dual overdrives, the 4:1 T-case gearing can make your vehicle very one-faceted.

Verne Simons

Tech Editor
I love low gearing; that is, high gearing numbers in axles and transmissions, such as 5.89:1, 7:1, and so on. Having said that, I do occasionally want to gear up, shift into Second—hell, maybe even Third—roll back a foot or so, and give ’er a good old-fashioned bump. For that very reason I like granny geared transmissions and not so much super-low-geared transfer cases. In fact, I once sold a Jeep after converting the T-case to 4:1. That, in my opinion, ruined that vehicle for me. Low was too slow even in Fifth gear, and high-range was too fast. I guess that means maybe what others see as good and low is a little bit too low for me and my driving style.

Now, a transfer case doubler and or reduction box bolted to a regular or even low-geared transfer case, like the ORD Magnum to a Ford NP205 in the 2018 Ultimate Derange Rover, is perfect. It offers a creepy crawly 5:1 when in low, low, but about 2:1 and 3:1 with only one of the low ranges engaged. Low-low is nearly useless for a good old-fashioned bump or for sand or deep mud, while being absolutely amazing for technical rockcrawling. But the other two ranges are perfect for when I want to bump it a touch or generate some wheel speed.

The best of both worlds is adding a planetary or other type of low-range box either between your transmission and T-case or in front of your transmission, like the absurdly strong Offroad Design Magnum Box system. The Magnum is a 2.72:1 planetary gearbox with a huge-by-ginormous input assembly for the accompanying NP205 T-case, allowing you to hammer the throttle with the planetary low engaged without fear of busting your T-case input.

Harry Wagner

Freelancer
Normally Verne Simons and I are on the same wavelength, but not when it comes to gearing. I had an NP435 in my Ford truck and I hated it. I have the same feelings about the four-speed SM420, SM465, and T18 transmissions. All of these are strong and compact and have a deep First gear, but they will make you think that you are driving a dump truck every time you have to shift! First gear was way too low for use on the street, requiring me to slip the clutch when I started in Second. And Fourth was direct (1:1), so the engine was wound out on the freeway. Yet somehow the gear splits seemed huge as well! I’ve since swapped out the NP435 for a five-speed ZF5, and I couldn’t be happier.

All of this was possible because I have an Offroad Design Doubler in the truck. I prefer dual transfer cases for gearing options on the trail, a modern five-speed transmission for the road, and matching the ring-and-pinion ratio to the tire size. If the gearing the transfer case is too low for a given situation, just shift to the next higher gear in the transmission and call it good.

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