Drivetrain Gearing Fundamentals
Building your 4x4 can be one of the most enjoyable and rewarding adventures that you will ever take. While we understand that not everyone is a master fabricator, simply getting to know the basics of how your rig operates will put you on the path to becoming an ace wrench. One of the keys to mastering the ins and outs of your 4x4 is grasping the numerous gear ratios that run throughout your rigs drivetrain. These are the ratios inside of your transmission, transfer case, and differentials.
Gear ratios are easy to follow, once you have the basic formula understood. For example; if you have a 5.13:1 differential gearset, it means that for every 5.13 times the pinion rotates, the wheel-end makes one complete rotation. In terms of a transfer case, a 4:1 ratio indicates that the transfer case output turns once for every four times that the transfer case input shaft rotates. It’s all about torque multiplication.
Each drivetrain component plays a vital role in how your rig performs both on- and off-road. One ratio that you will see frequently is the crawl ratio. The crawl ratio is a simple number that you can find by using the following equation (First gear ratio in the transmission x the transfer case low range ratio x the differential gear ratio). A numerically high crawl ratio number is great for rockcrawling, as it provides you with increased torque and control. For playing in places like the sand and mud, where wheelspeed is important, a higher crawl ratio isn’t as important or warranted.
Of course, the staff here at Four Wheeler each has their own crazy ideas of what gear ratios work best where. For that reason we’ve decided to break down each drivetrain component into its own category and let editors Cappa, Brubaker, and Mansour weigh in on each. Think you have a better drivetrain solution? Join the online conversation at www.fourwheeler.com.
I’m a manual transmission kind of guy in most cases. I enjoy being a part of the driving experience. I mean, if you were buying a sports car and you chose an auto, you’re kind of a weenie—even if it is a paddle shift. Selecting a manual transmission is very black and white for me. If the rig will see more street use than dirt, I like the modern five-speed overdrive transmissions, like the NV3550 and the NV4500. They are civilized enough to drive every day, yet still have a usable First gear for technical trail work and plenty of heft to keep from grenading when abused.
The extra Overdrive gear allows you to run slightly deeper axle gears, say in the 5.13 to 5.38 range with only 35- and 37-inch tires, without totally killing your highway speed and fuel economy. If my planned build is dirt-based I’m more likely to go with an old cast-iron truck four-speed with a really low First gear. The SM420 with its 7.02:1 First and the T-18 with a 6.32:1 are two of my favorites, but the NP435, SM465, and other similar truck transmissions are all viable options. With that super low First gear I’ve found I can get away with far less axle gear. In some cases, I’ve been able to make even 4.10 axle gears work with up to 37-inch tires both on- and off-road behind a stock V-8. Although, if I’m running a V-6, I’ll max out at a 35-inch tire with the 4.10s. If I’m eyeballing 40s or bigger with my V-6 and manual tranny I’m looking at 5.13 and deeper gears.
I drove a semi with a 10-speed manual transmission for a number of years, so I have no interest in rowing gears anymore. But if someone put a clutch pedal to my throat, I’d want a six-speed like the G56 found in the Ram 2500/3500 pickups behind the Cummins turbodiesel. With a First gear ratio of 5.94:1, I’d have big-time crawlability as well as the grunt to get going no matter what the grade/weight of trailer/load. The Sixth gear ratio of 0.74:1 would help make the rig highway-friendly.
I enjoy a manual transmission in most scenarios on- and off-road. I like the gearing options and find that the control and overall driving experience is better. In terms of manual transmission types, I am a fan of the NV3550 as it is a more versatile five-speed that can live behind mild V-8 power. I don’t care for four-speeds, as I view them as more limiting on the highway. A granny-low First gear is cool, but if I am spending most of my time tooling in the rocks, I am more apt to have an auto. If you’re running a naturally aspirated four-cylinder engine or anemic six-cylinder, a manual transmission can make the rig way more enjoyable.
If I go with an auto in my trail rig I want simplicity and something bombproof that’s inexpensive, so when it does smoke I can have it rebuilt without selling one of my 4x4s to pay for it. That means I’m sticking with the common older three-speed autos like the GM TH350, and TH400, Chrysler 727, and Ford C4 and C6. The four-speed autos are just too complex for my taste. That really limits my axle gear options if I want any kind of on-road speed, though. I’ve had good results matching the three-speed autos to 5.13 gears and 40-inch tires behind a V-8.
The rpm really winds up on the highway, but it’s doable for a rig that sees mostly trail use and short drives. I’ve also run unusual combos that sorta worked, including a stock V-8 backed by a three-speed auto, 37-inch tires, and pathetic 3.73 gears. It was a pig on-road, but since it was mostly an off-road rig for me, I found that shifting in the 2.61:1 low range made it livable in the dirt. When working with the more modern four- and five-speed overdrive transmissions I think deeper axle gearing is better. It helps compensate for some of the power loss and pathetic shift points many of these trannies have. Behind a V-6 and four- or five-speed auto I’ll opt for a gear that is typically one or two steps deeper than normal. For 35-inch tires I’ll look for 4.88 gears. For 37s I’ll aim for 5.13s or 5.38s.
Since I can only afford one 4x4 and that one vehicle has to do it all, I have embraced the new electronic automatic transmissions due to their incredible versatility. I want a respectable low First gear ratio for slow speed work and a healthy overdrive for highway driving. This is why I like transmissions such as the Allison 1000 six-speed. With a First gear ratio of 3.10:1 and a Sixth gear ratio of 0.61:1 it does a good job of covering all the bases. And the later models offer manual shifting for even more control without that infernal rowing.
Most modern automatic transmissions are downright scary. Many of the new autos come fitted with a “lube-for-life” faux dipstick cap, which means you can’t simply check the fluid after you submerge your wheeler. While the technology inside can be a bit overwhelming, the fact is that many of the new autos are pretty robust. While I agree with Cappa’s take on the classic three-speeds, if you are piloting a newer truck, you won’t be easily swapping transmissions without inheriting a barrage of dashboard lights and a wiring nightmare.
For automatic rigs I tend to overcompensate in the differential gearing department. For example, if the gear conversion chart suggests 4.88s to put the rpms back to stock, I will likely opt for 5.13s. This is especially helpful for those living in a mountainous area as it will keep your transmission from constantly searching. Once again, if you have a smaller engine, numerically higher gears will generally always be a good thing on- and off-road.