If you have ever driven a classic truck before, you may be surprised the first time you do. When placed in first gear, some classic trucks don’t crawl forward as fast as you’d expect. However much you lay on the throttle, nothing seems to change except the revs, which rocket sky-high as the truck groggily inches forward. You may even find that this gear is so weak that it seems like a better idea just to start the truck in second gear instead. This is not a transmission defect but is caused by special type of gear known as a granny gear, and it's there for a good reason.
Contrary to what its name implies, granny gears are actually quite strong and are necessary for trucks (especially heavy-duty ones). You might even recognize the term "granny gear" from what some people call the lowest gears on a bicycle (the ones that require such little effort to spin the wheels that anyone could ride it). Granny gears in truck transmissions are essentially the same thing. Without the proper first gear, these trucks would struggle to take off when hauling a load or towing.
Put most basically, a granny gear is a gear with a really high gear ratio. The standard ratio for the first gear in a transmission is anywhere from 2.00-3.00:1. A granny gear, on the other hand, can be twice as large. For example, a classic Ford Bronco with a T-18 transmission has a first gear ratio of 6.32:1. The engine has to spin a lot in order to produce just a single rotation of the wheels. Though not ideal for speed, this is fantastic for maintaining torque.
What Do Granny Gears Do?
Granny gears maximize torque at the expense of speed and fuel efficiency. In a sense, granny gears are the reverse of overdrive gears which have a gear ratio of 1.00:1 (or possibly even lower).
The NP435 transmission and the Borg-Warner T18 transmission are two transmissions found on Ford trucks that both came with a granny gear. On GM vehicles, the Muncie SM465 transmission comes with a granny gear as well.
Can You Skip Granny Gears?
Since granny gears have such absurdly high ratios when compared to most other gears, they aren't frequently used for normal driving. Given its slow speed potential, many drivers choose to skip first and start the car in second.
While this isn’t the worst thing that you can do to your transmission, it still isn’t recommended. By beginning to move your vehicle with the granny gear, if even just for a little bit, you save a nominal amount wear and tear on your clutch. Again, there are situations where starting out in second is ideal, but if you have no reason to, save your clutch and use first.
When Should You Use A Granny Gear?
There are several instances in which an especially tall first gear is useful. Any time you need a lot of torque, using a granny gear is a good option. There’s a reason they are referred to as “stump pullers” after all.
As mentioned above, it’s a good idea to keep your truck in its granny gear a little longer if you are towing or hauling any big loads. Getting a heavy truck moving from a stop takes a lot of torque, and having an ultra-low gear can help get it moving without shredding the clutch.
Additionally, granny gears are a useful tool for off-roading. Their low speed and high torque output are perfectly suited for rock crawling. Keep in mind this isn't the be-all-end-all way to take your truck off-road, but it can certainly help.
All in all, Granny gears are an interesting feature mostly exclusive to classic trucks. In a sense, the feeling of having to coax a truck forward is one of the quintessential classic truck experiences.