What is a secondary fuel tank?

A secondary fuel tank is an auxiliary reservoir that increases a vehicle’s fuel capacity beyond the primary tank, enabling longer range between refuels. It is often installed in addition to the main tank and can be found in cars, trucks, aircraft, boats, and some military vehicles, designed to integrate with the existing fuel system while meeting safety and regulatory standards.

Definition and purpose

In practical terms, a secondary tank stores extra fuel separately from the vehicle’s primary tank. It can be connected to the fuel line for topping up the main system or serve as a dedicated supply with its own pickup. Secondary tanks may share a common filler neck or have a distinct fill port, and they typically use the same fuel type as the rest of the system. The primary purpose is to extend usable range, enable longer missions, or provide contingency fuel for demanding operations.

How it differs from the primary tank

Unlike the main tank, the secondary tank is not usually the primary source during normal operation. It is designed to be intermittently accessed, replenished, or transferred to the main supply as needed, and it may have independent sensors, pumps, and venting to prevent cross-contamination and maintain stable pressure in the system.

Common configurations and uses

The following configurations are commonly used to add capacity across different platforms:

• Saddle tanks mounted along the vehicle’s frame rails, often used on heavy-duty trucks to maximize usable capacity without intruding on the cabin or cargo area.


• Under-bed, trunk, or behind-the-seat auxiliary tanks that provide extra capacity while keeping the primary tank in its stock location.


• Aircraft auxiliary tanks such as centerline or wing tanks that extend an airframe’s range between refuels.


• Bladders or flexible tanks deployed in boats, military vehicles, or temporary setups for ballast management and rapid deployment.


• Portable auxiliary tanks or removable jerry cans that can be added or removed for temporary missions or expeditions.

These configurations expand range and flexibility but require careful integration with the fuel system, including pumps, sensors, venting, and safety valves to prevent leaks, contamination, and vapor release.

Operational considerations and safety

Implementing a secondary tank involves several critical considerations to ensure reliability and safety across platforms.

1. Fuel compatibility and contamination control: always use the same grade as the primary system, and avoid cross-contamination between different fuels or additives.


2. Fuel pickup, transfer, and pump compatibility: ensure the pump and lines are rated for the extra volume, and that pickup points avoid air pockets and starvation.


3. Ventilation, vapor management, and emissions controls: proper venting prevents pressure buildup and fuel vapor release that could pose hazards or regulatory issues.


4. Safety and crash protection: tanks should be mounted to minimize rupture risk, with proper restraints and, in some designs, separate shutoff or isolation features.


5. Legal, regulatory, and inspection requirements: many jurisdictions regulate secondary tanks, including labeling, fill limits, and periodic maintenance.

Ongoing maintenance—regular inspection, leak checks, gasket replacement, and line integrity—helps ensure the secondary tank remains safe and functional over time.

Maintenance, installation, and regulatory considerations

Professional installation is often recommended because secondary tanks must be integrated with fuel lines, pumps, returns, vents, and sensors. Regular maintenance includes inspecting fittings for leaks, testing vent and fuel lines for integrity, and ensuring sensors and gauges accurately reflect the tank’s contents. Operators should be aware of local regulations that govern fuel storage, fill procedures, and disablement in case of transfer faults. In marine and aviation contexts, certified standards and inspections may apply.

Summary

A secondary fuel tank is an auxiliary reservoir designed to increase a vehicle’s usable fuel capacity, extending range and flexibility. Found in land transportation, aviation, and maritime applications, these tanks require careful design, proper installation, and ongoing maintenance to ensure safe operation and regulatory compliance. When used appropriately, they enable longer journeys, fewer refueling stops, and better mission adaptability without sacrificing safety.

What are the three types of fuel tanks?

Internal fuel tanks fall into three main categories – integral, rigid removable, and fuel bladders. Integral fuel tanks are part of the structure of the vehicle or aircraft, whereas rigid removable fuel tanks fit into a specially designed compartment and can be taken out for inspection, replacement, or repair.

Why would a truck have two gas tanks?

Some trucks have two gas tanks primarily to increase range and improve weight distribution, making them more practical for long hauls, towing, or working in remote areas. It was also often easier for manufacturers to install two smaller tanks than one very large one. 
Primary benefits

• Increased range: Dual tanks allow a truck to carry significantly more fuel, which means fewer stops at the gas station, especially on long trips or while towing. 
• Improved weight distribution: By splitting the fuel weight between two tanks, manufacturers can balance the truck's weight, which can improve handling and traction. 
• Easier installation: It was often simpler to design and install two smaller tanks than one large, complex tank that would require significant engineering to fit. 
• Durability and convenience: With two tanks, a truck has a reserve in case one tank is damaged, particularly from off-road impacts. Some systems allow drivers to switch between tanks, while newer systems can balance the fuel automatically. 

Why they are less common now

• Modern trucks are generally more fuel-efficient, reducing the need for such a large fuel capacity. 
• The extra weight of a second tank and its associated components (like a second pump, wiring, and fuel lines) can offset fuel economy gains. 
• Manufacturers now focus on other features, such as comfort and advanced technology, which are prioritized over dual tanks. 

What is a secondary fuel example?

Secondary fuel - Petrol, diesel, biogas.

How does a second fuel tank work?

Auxiliary fuel tanks work by storing extra fuel and transferring it to the main tank, either manually with a switch or automatically using a pump. When the main tank is low, the auxiliary system automatically starts a pump to refill it, shutting off when the main tank is full or the auxiliary tank is empty. Alternatively, a manual system requires the user to press a button or flip a switch to start and stop the fuel transfer. Some systems are also gravity-fed, where fuel flows from the auxiliary tank to the main tank as the latter is consumed.
 
This video explains the basics of auxiliary fuel tanks and what to look for: 57sPD Diesel PowerYouTube · Dec 15, 2019
How auxiliary fuel tanks work

• Automatic systems: These systems use a controller that monitors the fuel level in the main tank. When the main tank reaches a certain level (e.g., ¼ full), the controller activates a pump to transfer fuel from the auxiliary tank. The system automatically shuts off the pump when the main tank is nearly full or the auxiliary tank is empty. 
• Manual systems: These systems provide a dash-mounted switch or button. The user manually flips the switch to start the pump and transfer fuel when needed. A shut-off valve on the auxiliary tank can be used to stop the flow of fuel, and these systems are often designed to make it easier to track fuel consumption. 
• Gravity-fed systems: In this simpler setup, fuel is transferred to the main tank via gravity alone. The auxiliary tank is connected to the main tank and will continue to refill it as fuel is used. A shut-off valve is included to stop the flow as needed. 
• Safety features: Most systems have safety mechanisms to prevent overfilling the main tank. They also often have a shut-off valve, which is useful for maintenance or if there's a leak. Electrical systems require a fuel return line to prevent overfilling. 

You can watch this video to learn how to install an auxiliary fuel tank using the gravity feed method: 58sPat StewartYouTube · May 4, 2019
Installation and components

• An auxiliary fuel tank is physically installed and connected to a vehicle's existing fuel system. 
• Systems typically include the auxiliary tank, a transfer pump (if it's not a gravity-fed system), fuel lines, mounting hardware, and a control unit or switch. 
• Some systems include an in-cabin fuel gauge to monitor the auxiliary tank's fuel level. 

This video shows how to install a DeeZee auxiliary fuel tank in a truck bed: 52setrailerYouTube · Jan 17, 2023