How to Install an Auxiliary Fuel Tank in a Diesel Truck

Extending the fuel range of a diesel truck opens up possibilities for longer trips without frequent refueling stops. An auxiliary fuel tank mounted in the truck bed provides this capability through gravity feed or pump-assisted transfer systems. This approach suits truck owners involved in hauling, off-road adventures, or long-distance driving, potentially reducing downtime and fuel costs at remote locations.

Why Choose an Auxiliary Fuel Tank?

Auxiliary tanks increase total capacity by 30 to 100 gallons or more, depending on the model. Gravity feed systems rely on natural flow from the elevated bed tank to the main tank, simplifying setup without electrical components. Pump-driven options allow on-demand transfer, offering flexibility for varied driving conditions. Key advantages include fewer refueling interruptions, bulk fuel purchases at lower prices, and preparedness for areas with scarce diesel availability.

• Extended range: Up to double the distance between fills.
• Cost savings: Buy fuel in volume where prices are better.
• Convenience: Fill once for multi-day operations.

Assessing Your Truck and Needs

Before purchasing, evaluate bed size, payload capacity, and local regulations. Most systems fit full-size pickups like Ford F-250, Chevy Silverado 2500, or Ram 2500. Confirm the tank does not exceed gross vehicle weight rating (GVWR) when full—diesel weighs about 7 pounds per gallon. Check state laws on auxiliary tanks; some require DOT certification for highway use.

Decide between gravity feed for simplicity or electric pump for control. Gravity systems work best with the auxiliary tank higher than the main tank, ensuring complete drainage without air locks. Pump kits suit scenarios needing precise transfer rates or reverse flow prevention.

Gathering Tools and Materials

Essential tools include a drill with 5/16-inch and 3/4-inch bits, wrenches, hose cutters, measuring tape, marker, and safety gear like gloves and eye protection. Materials typically comprise the tank, neoprene strips for vibration isolation, hoses, clamps, shutoff valves, and mounting bolts. For pump setups, add wiring tools, fuses, and a transfer pump with pickup tube.

Planning the Installation Location

Position the tank forward in the bed, away from the tailgate for balance and access. Ensure tabs align without hitting frame components, wiring, or exhaust. Verify toolbox clearance if equipped. Mark front edge and four neoprene spots evenly spaced. This prevents shifting and protects the bed from wear.

Inspect under the bed for interferences. Drill cautiously to avoid wires or brakes. For gravity feed, align the drain with a bed side hole for hose routing.

Step-by-Step Gravity Feed Installation

Gravity systems prioritize simplicity and reliability for off-road or basic transfer needs.

1. Position tank and mark tab holes plus neoprene lines.
2. Drill 5/16-inch holes through tabs and bed; secure temporarily with one bolt.
3. Drill 3/4-inch hole for fuel hose near drain plug alignment.
4. Cut neoprene into four strips; apply sticky side down on bed high points.
5. Install shutoff valve in tank bottom with thread sealant.
6. Shorten fill hose by 1.5 inches; connect with rubber collars and clamps if needed.
7. Reposition tank over neoprene; bolt tabs securely.
8. Route hose from valve through bed hole to main tank adapter; clamp connections.
9. Check main tank vent; extend if open to prevent siphoning.

Test for leaks by filling partially and observing flow. Ensure downhill gradient avoids air pockets.

Electric Pump Transfer System Setup

For controlled transfer, integrate a 12V pump. These handle deeper draws and include auto-shutoff nozzles like gas station pumps.

• Assemble pump with telescoping pickup tube; extend to tank bottom.
• Apply Teflon tape counterclockwise on threads; hand-tighten plastic fittings.
• Mount pump securely near tank.
• Wire to battery with inline fuse on positive lead; use ring terminals.
• Connect discharge hose to fill neck; add filter if hauling dirty fuel.

Pumps enable filling on the fly via solenoid valves, ideal for hotshot trucking. Position fittings low on the main fill neck for full drainage.

Fuel Line Routing and Connections

Use fuel-rated hoses resistant to diesel degradation. Secure with stainless clamps. Route away from heat sources and moving parts. For gravity, maintain downward slope; cut hoses precisely. In pump systems, include check valves to prevent backflow. Verify fit before permanent install—some trucks need custom elbows.

Safety Measures and Best Practices

Never mount tanks to the frame; bed mounting preserves warranty and safety. Ground electrical components to avoid sparks. Install fire extinguishers nearby. Label the system clearly. Perform leak tests with soapy water before road use. Comply with FMCSA rules for commercial vehicles.

Troubleshooting Common Issues

Slow flow? Check for kinks or high points causing air locks. Leaks? Tighten clamps and retest. Pump failure? Verify fuse and battery connections. No transfer? Ensure pickup tube reaches bottom and vent is clear.

Maintenance for Longevity

Inspect hoses annually for cracks. Drain water from filters seasonally. Clean pickup screens. Monitor for corrosion, especially in salted road areas.

Cost Analysis and ROI

Kits range $500-$2000. Savings accrue from 5-10 cents/gallon bulk buys over 10,000 miles/year. Payback in 1-2 years for high-mileage users.

Frequently Asked Questions

Can I use this on gas trucks?

Primarily for diesel due to hose compatibility; check manufacturer specs.

Is wiring required for gravity feed?

No, fully passive.

How much range does 50 gallons add?

About 500-700 miles, varying by MPG.

Legal for DOT inspections?

Yes, if certified and secured properly.

Off-road only?

Many kits specify off-road; verify for highway.

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