8 Lesser-Known Facts About Semi-Trucks

Every day, hundreds of thousands of semi-trailer trucks make their way down America’s highways and interstates. While most passenger vehicle drivers are familiar with the sight of these 18-wheelers, relatively few people have any good idea of how they function, or why. Here are eight of the lesser-known facts about the nation’s semi-trucks.

1) How Semis Got Their Name

Curiously, semis are not named for the part of the vehicle that contains the engine and driver’s compartment. Instead, they get their name from the fact that they’re designed to haul separate trailer units that lack steering components and can’t move independently. The trailer portion of a semi-truck does come equipped with a powerful braking system. However, unless attached to the front tractor unit, this system remains in a locked position. When connected to a tractor, the lock releases automatically so the trailer can roll.

2) Why Semis Run on Diesel Fuel

Semi-trucks run on diesel fuel, not the gasoline used to power most passenger vehicles. This is true because a gallon of diesel holds more energy than a gallon of gas. In some cases, this increase in usable energy can translate into as much as a 30% increase in travel distance. Since semis often carry freight for hundreds or thousands of miles, the significant bump in efficiency can lead to a substantial reduction in operating costs.

3) Semi-Trucks Burn a Lot of Fuel

It’s a good thing that the use of diesel fuel makes semis more efficient, because the engines on these vehicles burn through fuel at a rapid pace. In fact, the typical semi-truck needs about six gallons of fuel just to travel a single mile. Why is the burn rate so high? A semi engine needs to generate a lot of energy so it can haul the extreme weight of the attached trailer and its cargo.

4) Semi Engines Can Run Continuously

The engines installed in semis are designed to run for extended periods of time. In fact, the only unavoidable reasons for shutting a big rig down are periodic maintenance and the down times between scheduled trips. However, the practice of leaving a semi engine running can lead to an enormous cost in wasted fuel, as well as unnecessary environmental damage. In addition, some jurisdictions have introduced anti-idle laws that make it illegal to keep a semi-truck running when not traveling down the road.

5) Semi Engines Can Log a Million Miles or More

The lifespan of a well-made passenger vehicle engine is somewhere in the neighborhood of 200,000-plus miles. In stark contrast, the typical semi-truck engine can easily reach and surpass the one million-mile mark before showing any sign of a need for an overhaul. This extreme durability is one of the primary reasons why America’s semis haul such a large amount of cargo each year.

6) Semi Engines Are Designed for Easy Rebuilding

When they do eventually fail, semi-truck engines are typically easier to rebuild than passenger vehicle engines. That’s because semi engine manufacturers design their products to make the rebuilding process as simple as possible. This efficient design principle makes it relatively easy to swap out parts subject to wear-and-tear-related damage, including pistons, bearings and connecting rods.

7) Not All Semi-Trucks Have the Same Number of Gears

The average semi-truck has 10 gears. This makes it possible to find the right transmission setting for most loads and road conditions. However, some semis have transmissions equipped with three, five or eight additional gears. The larger number of gearing options translates into an easier time finding the optimal settings for even the most unexpected or extreme driving situations.

8) Semi-Truck Mud Flaps Have Multiple Purposes

Naturally, the mud flaps installed behind the wheels of a semi-truck will help prevent mud and various types of road debris from creating a road hazard for people driving passenger vehicles. However, these simple devices also provide other important benefits. These benefits include:

  •         Keeping semis from pouring sheets of rainwater over nearby passenger vehicles
  •         Decreasing the amount of drag a semi produces when moving at highway speed

By reducing vehicle drag, mud flaps also make semi-trucks more fuel efficient.