FreightViking is reader-supported. We may earn commissions if you buy through our links. Also, as an Amazon Associate we earn from qualifying purchases.

How Do Semi Truck Fuel Tanks Work? (Explained & Solved)

Semi truck fuel tanks are far more complex than the average vehicle. While most vehicles have a singular tank, semi trucks have multiple. 

Maintenance and upkeep can be more involved, however, can save you thousands in the long run. 

Here is a quick answer to how Semi Truck Fuel Tanks Work:

Semi truck fuel tanks operate with multiple pumps of low and high pressure which moves the fuel from the tank through to the engine, thereby powering the truck. Typically there are 2 tanks on opposite sides of the truck with a balancing system to ensure equal weight distribution. 

How do Semi Truck Fuel Tanks work?

A semi truck fuel tank is typically a two-tank system. There is the primary tank and the secondary tank which works as a backup system to the first tank.

Although, the pump pulls from both sources to ensure a balanced weight in the truck.

When traveling long distances it can be difficult to stop and refuel constantly, let alone hope there is a place to refuel on your route. 

There are some other reasons for the two-tank system other than to extend the distance which can be traveled. Two are necessary in order to keep the truck balanced on the road, because one tank full on one side can actually lead to tilting or falling. 

Most tanks have an automatic balance while filling the tank which prevents one from being filled without the other being filled equally. The pump of the truck takes the fuel from the tank and powers the truck for thousands of miles. 

How much Fuel can a Semi Truck Fuel Tank hold?

The size of your truck can impact the amount of diesel it holds. Although, on average a semi truck diesel tank holds around 300 gallons of fuel.

Each tank holds about 125-150 gallons each, which averages out to about 300 gallons. 

This equals about 2,000 miles on a full tank of diesel if you are traveling at a relatively steady pace and even ground.

A semi truck traveling at an incline may only get about 2 miles per gallon, whereas on level ground they get around 6.5-7 gallons a mile. 

Remember when filling your tanks don’t fill them up all the way. Diesel fuel tends to expand when it is warmed and can overflow which can create many issues. There are EPA regulations on this so you want to be sure to prevent this. 

How many Fuel Tanks do Semi Trucks have?

Semi trucks usually have 2 diesel tanks on either side of the tractor. These get filled evenly while at the pump, but you must be sure the valve to the second tank is open when filling them. 

Usually, there is a self-balancing device within the tank that helps to ensure even distribution between them.

These tanks are usually mounted on the side with various braces, with 3 designs being the most common. 

These include D-style, cylindrical, and rectangular tanks. Each is selected for different reasons, while the rectangle holds more fuel and the cylindrical has more aesthetic appeal. 

The D-style was intended as a balance between the two. It is usually cylindrical in shape while holding substantial levels of fuel. These tanks are usually of aluminum or other light metals to prevent complications at the weighing station.

How do Dual Fuel Tanks in a Semi Truck work?

In a semi truck the tanks are usually located on either side of the truck and hold about equal amounts of fuel. There is a mechanism that connects them, which is usually a pipe. 

When one tank is nearly dry, the truck typically automatically switches from the empty tank into the secondary tank. It is not wise to allow the tanks to run dry, and rather let them get to about ¾ empty before refilling. 

Do you need to Switch between Tanks for a Dual Tank System?

Some trucks switch from one tank to another automatically, while others need to be switched manually.

This can vary based on the age of the truck or the model. Although, more modern models pull automatically from both to prevent the truck from tipping. 

You can typically switch between systems while you are driving your truck. It may be advisable to be careful while doing so because sometimes air buildup can cause the engine to sputter a bit. 

Does a Truck with 2 Gas Tanks have 2 Fuel Pumps?

Each tank typically has a pump of its own which enables switching between tanks while driving safer and easier.

This is usually done by having a small low-pressure pump and a larger high-pressure pump outside of the tanks.

If experiencing low fuel pressure, be sure to check the outer pump last, to ensure it is not either of the inner pumps that are not damaged.

It could also be the fuel filter, which is why it is really important to change them on a regular basis. 

Does the Semi Truck Engine draw Fuel from both Fuel Tanks?

This can depend upon the model of semi truck you drive, but usually, the engine will draw from both tanks at a time. Although, one is usually considered a backup tank, which is part of the reason why it will switch between them. 

Once the level of fuel gets to a certain level, right around ¾ or less, the pump draws from the second tank of fuel.

On average each tank, when completely filled, will last for about 1,000 miles a piece. This means that if both are filled you can get up to 2,000 or so miles of travel. 

It is important that the truck remains balanced, and the amount of fuel weighs quite a lot.

That is why it is important that the truck balances the distribution, and pulls from both while also ensuring the prevention of running out of fuel. 

What is Inside a Semi Truck Fuel Tank?

The interior of a fuel tank has a small, low-pressure pump that moves the fuel from the interior pump and into the outer high-pressure pump. These pumps work to move the fuel and balance the weight of the fuel in the truck. 

Maintenance of the interior of your fuel tanks is very important to maintain efficient and effective movement of your fuel.

Sludge tends to build up inside your tank, and if you rinse it regularly with warm water and appropriate cleansers, it can remove much of the sludge and promote efficiency. 

The pump also has filters that should be cleaned or changed when cleaning your tank. This helps to protect your engine from impurities and costs you even more money in fixing your semi truck. 

How does the Fuel Tank Sending Unit work?

The sending unit in a semi-truck works as a gauge to determine the fuel levels in each tank. They monitor the amount of fuel used, and how close each tank is to empty. There is an arm, float, and resistor in this system that works to keep a check on the fuel levels in your tank. 

If you are noticing the fuel gauge is inaccurate on your truck, or maybe it is no longer moving to show levels, you will need to check this system.

Such signs include erratic readings as well, with some assuming they have more fuel than they do. 

Or, if the gauge is inaccurate you might overflow your system and this can damage your truck or cause a spill. In order to prevent this, if you notice any inaccurate behavior from this system get it worked on right away. 

What Problems are associated with Semi Truck Fuel Tanks?

There are strict EPA regulations regarding the rates of refueling a semi truck tank. Because they are so large they are permitted to be filled at a quicker rate than a typical car; however, there are a lot more issues with a semi truck spill due to the volume of the spill.

Other issues are the inaccurate fuel readings discussed above which can result in damage to your engine if the fuel were to overflow or dry up.

Other signs of issues in your tanks can be overheating as a result of a bad pump and a lack of fuel getting to the engine. 

The best way to prevent any serious issues is to perform routine maintenance and pay attention to any warning signs. 

Is it Bad to Drive with your Semi Truck fuel Tank on nearly Empty?

It is very important that you do not allow your truck to run out of fuel.

This is due in part to the fact that if you are to run out of fuel miles from the nearest refueling spot, you may need to pay good money for a truck to come and refuel you. 

Other issues can lead to running the fuel pump dry. This is very bad for your pump, and may permanently damage it, which is not worth the risk or extra time bought. 

With that being said, you don’t want gunky old fuel sitting in your tank either. This is why it is best to allow the tank to get to around ¾ a tank before refueling.

Although, remember to know your route, as going uphill burns far more fuel than downhill or level ground. 

At the end of the day, it is better to be safe than sorry. If you aren’t sure if your truck will make it to the next stop, refuel while you can.

You can always perform maintenance on your truck to clean out the fuel tanks.