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What Is a Granny Gear Transmission?

The 2021 Bronco seven-speed throws down the granny gear gauntlet with the first true crawler gear manual transmission an SUV since 1991.

Christian Hazel
Jul 17, 2020
What is a granny gear transmission: a question a child might ask but not a childish question. Opinions vary on what constitutes a "granny tranny," but in a nutshell it's a manual transmission with an extremely deep First gear ratio that allows the vehicle to crawl along at a super-slow pace akin to that of a kindly old granny shuffling in absolutely no rush to Sunday services. These transmission types were super common back in the early/mid-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 First and a 2.33:2 Second) constitutes a granny gear transmission, but we don't really buy that. To us, a granny gear is something that's otherwise useless in normal, everyday driving. You're not going to use the 6.68:1 First gear in your 1978 Bronco's NP435 every time you pull away from a stoplight, only to bang solidly down several flights of stairs to a Second gear of 3.34:1, are you? Nope, we didn't think so. And it's for that logic that we immediately discount gearboxes like the NV3550, AX15, T5, and even the NSG370 six-speed with its 4.46:1.
 

Necessity of a Granny Gear Transmission?

Photo 2/6   |   02 Old Truck Require Granny Gear Transmisison
With relatively anemic low-compression four-, six-, or eight-cylinder gas or diesel engines, these absurdly tall First gear ratios between 6.0 and 7.02:1 helped get a GVW 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. Or, it made it possible for these low-horsepower, low-torque engines to pull loads up steep hills without overwhelming the drivetrain, stalling the engine, and letting the whole shooting match careen back down with 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-'80s -ton SUV could more than double that of a 1950s over-the-road semi tractor, and both the need and desire for a super-deep First gear ratio was waning. So as the OEs began using up their supply of granny gear four-speed manuals in SUVs, they began transitioning either to manuals that offered closer-ratio First gears or omitted the manual option altogether in favor of overdrive automatic transmission.
 

The Last SUV Granny Gear

Photo 3/6   |   03 Gm Sm465 Granny Gear Four Wpeed Manual Transmission
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 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 S10 Blazers, Jeep Wrangler, Cherokee, Grand Cherokee (at least for one year), and Ford Explorer all used aluminum 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. Dodge used the NP435 four-speed in its Ramcharger from the unit's introduction until roughly 1990. We've only heard anecdotal incidences of close-ratio NP445 manuals in 1991 Ramchargers, and by the Ramcahrger's final year of UA production, the A-535 five-speed was the manual option. GM was perhaps the last to offer a granny gear transmission with its 6.55:1 First geared SM465, which was in use through the 1991 model year. But even by 1989 these manual transmissions were exceedingly rare in Blazer and Suburban SUVs, and although they technically built them, today you'd be hard-pressed to find a 1990-1991 Blazer or Suburban sporting a factory SM465.
 

The First Granny Gear SUV Manual Since the Last Granny Gear SUV Manual

Photo 4/6   |   04 Ford Bronco Off Road
So given that Jeep hasn't used a real granny gear manual transmission since it discontinued the use of T18 and T98 transmission 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, then we're here to crown the 2021 Ford Bronco as the last SUV to offer a true granny gear manual transmission. The 7MTI550 seven-speed manual transmission option in the 2021 Ford Bronco features a crazy 6.58:1 "Crawler" gear. That's what they call it, "Crawler" gear. 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 cast-iron four-speeds and trumps most of the five- and six-speed manuals that - and 1-ton pickup buyers have had to choose from in the past few years.

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
Check out the gear ratios on the new Ford Bronco's 7MTI550 and see for yourself if you agree that it's the first granny gear transmission since the last offered in an SUV almost 30 years ago. So now it's Wrangler's turn. The granny gear gauntlet has been thrown. Whatcha gonna do, Jeep?
Photo 5/6   |   05 Sm420 Granny Gear Transmission
Photo 6/6   |   06 Jeep T98 Granny Gear Four Speed Manual Transmission

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