Skip to Content

How Many Speeds Does a Semi-Truck Have?

How Many Speeds Does a Semi-Truck Have?

Semi-trucks have a tractor with separate trailers to haul heavy loads. They differ from small vehicles in many ways. They have a more powerful engine and more tires than a small car. In the same way, the number of speeds also differ from the rest of the automobiles.

How Many Speeds Does a Semi-Truck Have? A semi-truck can have up to 18 speeds, but usually, they have 10 gears. It has three types of transmission manual automatic and automated manual. The large number of gears helps to get more power within the specified RPM to control the speed and torque of the vehicle.

The speeds are an essential component of the truck for its better performance and stability. All types of automobiles have both forward and reverse gears to control the speed and direction of wheels.

What do the speeds do in semi-trucks?

Gears are a mechanism in a vehicle that are used to transfer rotary motion and power to the wheels and other parts of the automobile. They are also part of other machines to control the movement.

They are used to speed up or slow down the semi-truck. They also have roles in changing direction in the truck. They are connected to the axles, and the engine rotates a series of speeds in the semi-truck’s transmission to control the wheel’s speed and direction.

They have teeth around the edges that prevent them from slipping. The speeds in any vehicle are essential to change and control the speed. They have teeth that interlock with each other to transfer power and change the speed.

They also deliver extra force to the wheels by spinning faster to increase the speed and rotational motion of the wheels.

Number of speeds in a semi-truck

Gears in a vehicle is determined by the types of vehicle transmission and the engine’s power. A typical small vehicle, like a car has 5 to 6 or 7 speeds. But big trailers are large vehicles with more powerful engines and heavy-weight cargo.

They have a large more gears to control the speed and direction. Typically a truck has 9 to 10 speeds, but semi-trucks can have up to 18 gears. Some of them can have 14 or 15, depending on the model and manufacturer of the vehicle.

These gears are essential to haul heavy objects from inclines and terrains. Commonly semis have ten forward and two reverse gears.

The speeds or gears are divided into three categories:

  1. Lower gears (1-4) to start and pull heavy objects
  2. Intermediate gears (5-10) to attain moderate speed on uphill. They also have a role in maintaining balance and momentum on high hills.
  3. Higher gears (11-18) are used to get the top-level speed and descend down from the hills.

Types of transmission in a semi-truck

A heavy semi-trailer has three types of transmission. I have explained their details below.

Manual transmission

In this transmission, the driver can control the gears by pressing the clutch pedal. A truck with manual transmission can have up to 18 speeds and unsynchronized transmission.

This type of transmission also has three pedals to control the gears. In the manual system, the slider gears are placed between the transmission to shift and engage speeds. It is difficult to learn but has better fuel economy.

Automatic transmission

In this type of transmission, the trucks use a computer system to change the gears. It contains only two pedals as compared to the manual system.

There are fewer speeds in this transmission is lower than the manual system. The automatic transmission is further divided into two categories.

Automatic planetary transmission

This type of transmission easily synchronizes the engine and drive rotation. It involves coupling the engine with the driveline. It is the most common type of transmission found in semi-trailers. It is easier to learn, but the drivers require training or a license to use it.

Dual-clutch transmission

This type of transmission is computer controlled and uses two coaxial shafts and two clutches. It has one clutch to control odd gears and another to control even gears.

Automated manual transmission

It is a combination of automatic and manual speeds In the trucks. This transmission has electrohydraulic or electromechanical actuators to select the gears automatically. This type can tackle heavy traffic and more fuel efficient than other systems.

How to know the total number of speeds in a semi-truck?

The number of gears depends upon the type of transmission. You should check the type of transmission before searching for the speed pattern.

In a manual transmission, they are present on the gear shift. While in an automatic transmission, they are present above the front window of the big trailer.

Why do semi-trucks have so many speeds?

The number of speeds in heavy trucks are more than in small vehicles. There are various reasons for having additional gears in semi-trucks. Here is a brief list of all of them.

Size and power of the engine

The engine and vehicle size can determine the number of speeds in automobiles. They are large vehicles and carry heavy loads.

They can weigh approximately 36000 pounds. It is a big load and requires more gears to haul such heavy loads smoothly.

The size of the engine also impacts the number of gears in trucks. The engine present in the semi-truck is large and can produce an enormous torque to run the wheels.

Narrow power band

An important reason for having so many gears is the narrow power band of the engine for each speed. It means each speed has a specific range of RPM that is effective.

The semi-truck engine can operate at its specified RPM by using different gears. They enable the vehicle to gain more torque and power within the same RPM. Otherwise, the engine has to consume a lot of fuel to deliver high torque and power to the wheels.

Having more gears makes the vehicle fuel efficient and reduces the normal wear and tear on the truck’s part. Shifting gears allows the semi to maintain its torque and speed within the specified power band and RPMs.

Better control over speed and power

These trucks have approximately 18 speeds, and each of them has a specific purpose. These trucks have to haul heavy objects and thus need to accelerate and decelerate the vehicle more often.

A large number of gears in a semi-truck gives the driver liberty to control the speed and revolution of the wheels while hauling the loads from inclines and terrains.

They ensure that the trailer is running smoothly under different conditions. The drivers have various options regarding the gear range to control the truck’s performance. It is better to drive safely on uneven roads and descend the trailer from a rocky hill.

Starting the vehicle at first gear can deliver enormous torque to the wheels. It comes with the possibility of the wheel spinning, which is a disaster.

The driver starts from the second gear to start the wheels slowly. That’s why having a wide range of gear ratios is better for the safety and performance of the trucks.

Related Articles:

Number of brakes in a semi-truck

What Year Nissan Hardbody Parts Are Interchangeable?