The Granny Gear Transmission Explained, Featuring the 2021 Ford Bronco
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The 2021 Bronco's seven-speed marks the first true in-transmission crawler gear in an SUV since 1991.
What is a granny gear transmission? It's a question many off-road neophytes (or even some with more experience) might ask, especially as the 2021 Ford Bronco has brought the relatively dormant terminology to the fore. Indeed, the new SUV's standard seven-speed manual transmission is really a six-speed with a seventh "granny" gear. So, let's dive in.
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What Is A "Granny Gear?"
Opinions vary on what constitutes a "granny transmission," but in a nutshell it's a manual transmission with an extremely low gear ratio that allows the vehicle to crawl along at a super-slow pace akin to that of a kindly old granny shuffling along. These transmission types were more common back in the early and mid 20th century, especially in utilitarian trucks.
Some may argue that any first gear ratio that's more than a full point from the next closest gear (for example, an NV3550 with a 4.01:1 ratio being its lowest, and therefore "first" in the sequential sense, and 2.33:1 being its next-lowest, or "second" gear) is what constitutes a granny gear, but we don't really buy that. Instead, a granny gear is something that's otherwise useless in normal, everyday driving. For example, you wouldn't use the ultra-low gear ratio as the transmission's "first" gear every time you pull away from a stoplight, only to bang solidly down several figurative flights of stairs to a much-taller (numerically lower) ratio? Nope, we didn't think so.
Why Use A Granny Gear Transmission?
With relatively anemic low-compression four-, six-, or eight-cylinder gas or diesel engines, early commercial trucks and utility vehicles used absurdly low gear ratios between 6.0:1 and 7.02:1 to help get their GVWs of 10,000 to 30,000 pounds up and moving well enough that second gear could be double-clutched on the fly and the load could continue down the road. It also made it possible for these low-horsepower, low-torque engines to pull loads up steep hills without overwhelming the drivetrain, stalling the engine, and sending the whole shebang careening backwards, guided only by gravity.
But as time and technology advanced, so, too, did the expectations of manual-transmission drivers. Consider that the horsepower and torque level of a late-1980s half-ton SUV could be more than double that of a 1950s over-the-road semi tractor, largely negating the need for a super-deep first gear ratio. So, as automakers began using up their supply of granny-gear four-speed manuals in SUVs, they began transitioning either to manuals that offered closer first and second gears or omitted the manual option altogether in favor of overdrive automatic transmissions. Oh, and two-speed transfer cases with low-range gearing also largely obfuscate the need for a granny or crawler gear built into the transmission.
The Last Production SUV With A Granny Gear Was…
We'll preface this by saying granny gear transmissions have never really gone away. At one point in time, GM, Ford, and Ram all offered five- or six-speed manual transmissions with a low first gear ratio somewhere in the 5:1 or 6:1 neighborhood in pickup trucks. But by the time the most commonly known variant of these, the NV4500, began appearing in GM trucks in 1992 and Dodge trucks in 1995, SUVs had more or less given up the option of a manual transmission. The small SUVs like GM's S10 Chevy Blazer; Jeep's Wrangler, Cherokee, and Grand Cherokee (at least for one year); and Ford's Explorer all used aluminum-housing four-, five-, or six-speed manuals of one form or another that didn't feature a granny gear. Ford used the NP435 with a 6.68:1 first gear ratio up through the 1986 model year, but for 1987 the four-speed manual was dropped and a Mazda-sourced M5OD close-ratio five-speed was the manual offering.
New Granny Gear SUV Manual, Who Dis?
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Jeep hasn't used a real granny-gear manual transmission since it discontinued the use of T18 and T98 gearboxes decades ago, and—if you can even find one—the last SUV with a granny gear transmission was arguably the 1991 Chevy Blazer with an SM465. So the 2021 Ford Bronco is resurrecting a long-dormant tradition, and immediately becomes the only current SUV to offer a true granny-gear manual. The 7MTI550 seven-speed manual transmission option in the 2021 Ford Bronco features a crazy low 6.58:1 "Crawler" gear, as Ford refers to it, but we really know that it's a granny gear in the old-school parlance. The 6.58:1 ratio is every bit as deep as those granny-gear ratios from the golden age of the gearbox type and beats most of the five- and six-speed manuals that three-quarter and one-ton pickup buyers have had to choose from in the past few years. Oh, and the Bronco's overall ratio can be much, much lower, thanks to its standard part-time four-wheel drive with a two-speed transfer case. Drop the Ford into low range, and boy, it'll put on its slippers, grab its walker, and shuffle right along to Sunday service.
The transmission, though, is just the first rung on the gearing ladder, especially on a 4WD vehicle. Next comes the transfer case, which multiplies the gearing when it's engaged, then the axles, which multiply the gearing, too. Depending on the ratios in each component, a granny gear can be essential or meaningless by the time the power gets to the ground. For example, if a manufacturer spec'd a short granny gear but then paired it with tall transfer case and axle gears, the mechanical advantage of the granny gear would be nullified and you'd get the same end result as normal first gear and shorter transfer case and axle gearing.
In the new Ford Bronco's case, it offers both ways. The granny gear has a very short 6.59:1 ratio and the axles can be equipped with either 4.46:1 or 4.70:1 gears. That's all very short and very much in the granny gear ethos, but the actual crawl ratio depends on which transfer case you order. Choose the standard electronically controlled transfer case with its 2.72:1 gearing and you get a 79.92:1 crawl ratio in 4-Lo with the granny gear. Select the optional electromechanical transfer case, though, and it's 3.06:1 gearing gets you a crawl ratio of 94.75:1 in 4-Lo with the granny gear.
Is that good? The obvious comparison is a manual-transmission Jeep Wrangler, which doesn't use a granny gear. Instead, Jeep uses a shorter first gear (though at 5.13:1, not nearly as short as Ford's Crawler gear), a 2.72:1 ratio in the transfer case, and a 3.45:1 axle ratio for a standard crawl ratio in 4-Lo and first gear of 48.13:1—way shorter than Ford's granny gear set-up, but only slightly taller than the crawl ratio you get with the Ford in first gear instead of Crawler (51.92:1). Order a Wrangler Rubicon, though, and you get a 4.0:1 transfer case gear and 4.10:1 axle gears for a crawl ratio of 84.13:1, splitting the difference between Ford's two granny-gear options.
The takeaway: Every manual-transmission Bronco will have seriously short crawl ratios from the factory, much more so than the manual-transmission Jeep Wrangler, which only has a comparable crawl ratio on the Rubicon model.
2021 Ford Bronco 7MTI550 Specs
7-speed (6+1 crawler gear) Getrag manual
Offered on 2.3L engine only
Crawler - 6.588:1
1st - 4.238:1
2nd - 2.365:1
3rd - 1.453:1
4th - 1.000:1
5th - 0.776:1
6th - 0.646:1
Reverse - 5.625:1
SOURCE: MOTORTREND