Auxiliary tank and transfer tank get used as if they mean the same thing. They do not, and the difference matters because it changes what the tank is certified for and how it is allowed to be plumbed.
What Is the Difference?
A transfer tank carries fuel in the bed and pumps it out to something else, usually equipment on a job site. An auxiliary or refueling tank is connected to the truck's own fuel system so the fuel it holds feeds the engine and extends range. Same tank shape in many cases, different plumbing and different rules.
Transfer Tanks Are the Simpler Purchase
A transfer tank is a self-contained item. It sits in the bed, it is strapped or bolted down, and a twelve volt pump moves fuel out through a hose and nozzle. Nothing touches the truck beyond a fused power feed for the pump.
That is why the majority of tanks sold are transfer tanks. Fitting one changes nothing about the vehicle, and taking it out again leaves the truck as it was.
The pump is the part that decides how useful the tank is day to day. A twelve volt pump moving around 15 gallons per minute fills a machine quickly enough that nobody stands around waiting, and it runs off the truck battery through a fused circuit. Some tanks ship with a pump fitted and some are sold bare, so confirm which you are ordering.
Auxiliary Tanks Involve the Truck's Fuel System
Feeding a tank into the factory fuel system is a different job. It has to be done with a kit designed for it, plumbed so the auxiliary tank drains into the main tank rather than into the engine feed directly, and set up so it cannot overfill.
- The tank has to be rated for the fuel, and most bed tanks are diesel only.
- Venting has to be correct or the transfer stalls partway.
- Fuel gauging becomes two readings instead of one.
- Emissions and roadworthiness rules apply, and they vary by state.
If range is the goal, look for a tank and kit sold specifically for refueling rather than adapting a transfer tank to the job.
Weight Applies Either Way
Diesel weighs roughly 7 lb per gallon. A 50 gallon tank is about 350 lb of fuel, and a 100 gallon tank is about 700 lb before the tank itself is counted. That load sits high and forward in the bed, which is the part of the truck least able to absorb it. Check the payload rating before choosing a capacity, and secure the tank to the bed rather than trusting a strap.
It also moves. Fuel sloshing in a part filled tank shifts weight under braking and through corners, and a tank that is only held down by friction and a ratchet strap will eventually prove it. Use the tank's own brackets or a bolt-down kit into the bed floor.
Picking the Right One
If you are fuelling machines, a transfer tank with a pump is the answer and the install is straightforward. If you are chasing range between fill-ups, you want a refueling setup and a fitter who has done one before. Either way, start from shape and capacity in transfer tanks, and see truck tool boxes if secure storage has to share the same bed.