Why do Semi Trucks Have so Many Gears?

In the world of cars and trucks, most vehicles have either an automatic or manual transmission with anywhere from three gears up to ten gears. As transmissions have become more advanced, it’s becoming more and more common to see things like a ZF 8-speed being used in a performance application or even the new 10-speed transmission in something like the Ford F350.

But, in the world of commercial trucks, things can be quite a bit different, with many semi-trucks using manual transmission with anywhere from 10 to 18 forward gears. But the question is, why do semi-trucks have so many gears, and is there any other option or are they just going to be like that forever? So, let’s get into it and break it down.

Roll intro

Right off the bat, I want to get straight to the point and say the reasons for using a manual transmission with a ton of gears are very simple: one, to keep the engine in its optimal power range, and two, torque multiplication. But, as you’ll see as we go through this, it’s a little more complicated than just that.

Narrow Powerband

So, starting this off, let’s dive into the first reason for using a bunch of gears on a semi-truck and that’s the super narrow powerband of these trucks. I won’t go into crazy detail about why these trucks have narrow powerbands because we covered a lot of that in our other article covering why semi-trucks use inline-six engines, but to briefly cover some of it:

With a low revving engine, there’s reduced stress on basically everything and it also helps to eliminate any complicated parts and allows the engine to be incredibly simple. Semi-truck engines generally make insane amounts of torque through a long stroke as well as other design features, but with a super simple engine that’s specifically designed to last upwards of one million miles or more, plus the long stroke, these engines generally have very low rev limiters, which also means they have narrow power bands.

With many trucks, you’re seeing peak power from 600rpm to 1600rpm or 1000rpm to 1500rpm. It’s different for every truck depending on the engine, but simply put, there’s typically only a couple of hundred RPM of peak usable power and efficiency, so you need to keep the engine within that range.

To stay in the range, though, that means you need a lot of transmission gears. Say for example you only had a few hundred RPM of usable power mated with a three-speed transmission. By the time you shift from 1st to 2nd or 2nd to 3rd, you’ll be putting the engine outside of its peak powerband, which means you’ll lose power and probably lose speed.

So, by using a transmission with 18 forward gears, you can constantly keep the engine in that narrow powerband for improved acceleration and fuel efficiency. You also have to remember that with lower engine operating speeds comes reduced fuel consumption and improved emissions output.

Torque Multiplication

That brings me to the second point, which is torque multiplication. And for those who don’t already know, let’s quickly take a look at an extreme theoretical example of torque multiplication of a lawnmower engine pulling a train. We only have a couple of horsepower to work with and let’s just say 100,000lbs of weight to make this simpler.

If we hooked the engine to the train at a high gear ratio, let’s say 1:1 for example, the engine would immediately die once it was loaded up, as it doesn’t have the power to move something that heavy at that kind of gear ratio, but if take this to the other extreme and use something like 100:1 gear ratio, the engine won’t stall once loaded up and will be able to move that heavy load, however, it will be incredibly slow.

With that in, we can look at something like an Eaton 10-speed transmission, where we can see the first gear is listed a 14.8:1, then 2nd at 10.95:1, then 3rd at 8.09:1, and so on, until the 10th gear at 1:1. With a lower gear ratio like 14.8:1, the torque of the engine is multiplied before it reaches the wheels and gives the drives wheels effectively more torque than what the engine is actually outputting.

But, the downside of this is that a low gear ratio is also very slow. In some applications, first gear can only get you to a couple of miles per hour, but really that’s all you need. The hardest part of driving a semi-truck is getting it moving from zero miles per hour. Once it’s moving, it’s easier and easier to keep it moving, and as such, the gear ratio goes lower and lower and lower to decrease effective torque at the wheels while also increasing drive speed to allow the truck to get to highway speeds.

With that in mind, it’s easier to understand why having more gears is better for an application like a semi-truck because with more gears comes less difference in torque multiplication from one gear to another. What I mean by this is that the percentage difference in torque multiplication between gears is reduced when you have more gears, which means simply you have more pulling power all the way to your final as compared to a transmission with fewer gears.

Improved Deceleration

That also works in the other direction, where engine braking or jake braking is very important on these trucks, as you generally want to use the actual brakes as little as possible, especially on a hill descent when loaded to reduce wear and tear on the truck and also prevent your brakes from overheating which can cause them to fail entirely.

By having more gears to pick from, you can effectively engine brake harder as compared to a transmission with fewer gears. And really, this is just torque multiplication backward, where first gear and last gear will have the same engine braking strength on something 10 speed vs an 18-speed assuming first and last gear have the same ratios, but all the gears between will offer better engine braking on the 18-speed because of reduced percentage change in gear ratio from one gear to another.

And before we move on to the next section, I’d like to mention that as the truck is set up for longer and heavier loads, you’ll generally find more gears on it. It’s not always the case, but as the loads get heavier and the drives get longer, there’s a clear trend toward using 18-speed transmission as compared to a 10, 12, or 15-speed transmission.

What About Other Drivetrains?

So, with torque multiplication and the narrowband in mind, why not use a transmission that keeps the engine in the correct powerband at all times, such as a continuously variable transmission? And the answer to that is actually pretty simple, and that’s reliability. While technically a properly designed CVT could work on a semi-truck, there’s nothing out there currently that fills that gap.

There have been developments towards a semi-truck CVT transmission, but the big thing holding it back is reliability. These trucks make decent amounts of power and have to haul very heavy loads, which means the load on the drivetrain is pretty insanely high and a CVT would likely end up burning up from the sheer load on the drivetrain.

That being said, CVTs have been used in heavy-duty applications plenty of times before. They are tanks with CVTs, tractors with CVTs, and so on. But for a truck that needs to reliably haul upwards of 100k lb loads all across the US or whatever country you’re in, a CVT generally is not going to give the driver the reliability they’re looking for.

With that in mind though, there is another option that has been gaining popularity which is an automatic transmission. Historically, these weren’t used because they simply couldn’t the drivetrain loads while also delivering excellent reliability, but in recent years that’s begun to change.

For example, the Kenworth T680 is one of the trucks that are now available with an automatic transmission. Some of the automatic transmissions on the market work simply as automated manual transmission and some of them are actually designed as a true automatic transmission from the ground up.

Understandably, the auto transmission is picking up in popularity, as it makes the trucks much easier to drive, which then opens the doors to more drivers and pretty much completely eliminates the injuries that can happen to long-term over-the-road truck drivers, such as their rotator cuff in their shoulder getting messed up from shifting so often every single drive for years or decades on end.

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