If you're tired of stopping every couple of hundred miles to fill up, installing an f350 aux fuel tank is probably the best move you can make for your truck. It's one of those upgrades that you don't think you need until you're deep in the middle of nowhere, towing a massive fifth-wheel or a heavy gooseneck, and watching that fuel needle drop way faster than you'd like. Let's be real: as much as we love the Power Stroke's torque, it can be a thirsty beast when it's working hard.
The stock tank on a Ford F-350 is decent enough for daily driving, but it starts feeling really small the moment you hook up a trailer. Depending on your wheelbase and year, you might have anywhere from a 34 to a 48-gallon tank. That sounds like a lot until you're getting 9 or 10 miles per gallon while hauling. Suddenly, your usable range is barely 300 miles before you're sweating, looking for a diesel pump that's actually easy to pull a 40-foot rig into.
The Problem With Stock Range
We've all been there. You're making good time on the interstate, the traffic is flowing, and you really don't want to stop. But the computer says you've got 50 miles to empty, and the next "easy access" truck stop is 60 miles away. It adds a level of stress to a road trip that just doesn't need to be there.
An f350 aux fuel tank basically deletes that anxiety. By adding another 40, 50, or even 60+ gallons of diesel, you're looking at a total range that can easily exceed 800 or 900 miles. Think about that for a second. You could drive almost an entire day without having to hunt for a gas station. You can bypass the overpriced stations near the city and wait until you find a high-speed pump at a dedicated truck stop where the fuel is cheaper and fresher.
Choosing Between In-Bed and Replacement Tanks
When you start looking into adding fuel capacity, you've generally got two ways to go. You can either swap your factory tank for a larger one, or you can bolt an auxiliary tank into the bed of the truck.
Replacement tanks are great because they don't take up any bed space. Companies like S&B or Titan make these massive mid-ship tanks that replace the factory plastic one. You get a much larger capacity—sometimes 60 gallons or more—and your fuel gauge usually works just like it did before (though you might need to recalibrate the "miles to empty" setting with a scan tool). The downside? They can be a bit of a pain to install since you're working under the truck, and they're generally more expensive.
On the other hand, a dedicated f350 aux fuel tank that sits in the bed is usually the more popular choice for serious haulers. These are often aluminum or steel boxes that sit right behind the cab. Yes, you lose about a foot of bed space, but the ease of installation and the sheer volume of fuel you can carry often make up for it. Plus, if you ever sell the truck, it's a lot easier to take the auxiliary tank with you to your next rig.
Gravity Feed vs. Pump Systems
If you go with the in-bed f350 aux fuel tank, you have to decide how the fuel actually gets into your engine.
A lot of guys go with a gravity feed system. It's the simplest way to do it. You install a "T" fitting into your factory filler neck, and the fuel literally just drains from the bed tank into the main tank as you drive. It keeps your main tank full until the aux tank is empty. It's cheap, it's reliable because there are no moving parts, and it's relatively easy to install. The main thing to watch out for is making sure you have a high-quality rollover valve and a manual shut-off, just in case.
Then there are transfer pump systems. These use an electric pump and a switch in the cab. When your main tank gets low, you flip a switch, and it pumps fuel from the back into the main tank. This gives you more control, and you don't have to worry about the main tank potentially overflowing if a valve fails (though modern kits are pretty good at preventing that). Some of the high-end systems even have digital controllers that handle the transfer automatically based on your fuel level.
Why Materials Matter
You'll see tanks made of aluminum, steel, and even heavy-duty poly. Most guys go for aluminum because it's lighter and won't rust. Diesel fuel can hold moisture, and the last thing you want is a steel tank rusting from the inside out and sending flakes of metal into your high-pressure fuel system. That's a nightmare you want to avoid at all costs.
If you do go with steel, make sure it's powder-coated on the outside and treated on the inside. But honestly, for an F-350, a diamond-plate aluminum tank just looks right. It matches the ruggedness of the truck and holds up to the weather if you don't have a bed cover.
The "Hidden" Benefit: Saving Money
Believe it or not, an f350 aux fuel tank can actually pay for itself over time. If you're a long-haul driver, you know that fuel prices can swing wildly from one state to the next. With an extra 50 gallons on board, you have the freedom to drive right past the expensive stations.
I've seen price differences of 40 or 50 cents per gallon just by crossing a state line. When you're buying 80 gallons at a time, those savings add up fast. It also gives you the ability to fill up at home where you know the fuel is good, rather than risking a "bad batch" of diesel at some sketchy station in the middle of nowhere just because you were running on fumes.
What About Installation?
Can you do this yourself? Absolutely. If you're handy with a drill and some basic hand tools, installing an f350 aux fuel tank is a solid Saturday morning project. You'll need to drill a couple of holes in the bed to bolt the tank down and one or two for the fuel lines.
The trickiest part is usually tapping into the filler neck. You have to cut a small section out of the factory rubber hose and hose-clamp the "T" fitting in place. It's a bit messy, but not overly complicated. Just make sure you've got a good seal; nobody likes the smell of diesel leaking every time they go around a sharp turn.
If you aren't comfortable cutting into your truck's fuel system, any local diesel shop can knock this out in a couple of hours. It's a very common modification for the Ford Super Duty platform, so most mechanics have done it a dozen times.
Is It Worth the Bed Space?
This is the one thing people get hung up on. If you have a short-bed F-350, giving up 12 to 18 inches of your bed can feel like a lot. If you also have a fifth-wheel hitch back there, things start getting crowded.
However, many manufacturers make "L-shaped" tanks or low-profile tanks designed to fit under a tonneau cover or around a hitch. You can even find combo units that are part tool box, part fuel tank. These are awesome because they give you a place to store your straps, tools, and hitch accessories while still giving you that extra range.
Final Thoughts
At the end of the day, an f350 aux fuel tank isn't just about carrying more fuel; it's about freedom. It's about not having to plan your entire trip around where the next Pilot or Flying J is located. It's about being able to take the scenic route without worrying if there's a station that can accommodate your 13-foot-tall trailer.
If you do any kind of serious towing or long-distance traveling in your Ford, this is one of those upgrades that will make you wonder how you ever lived without it. It turns the F-350 from a great truck into a true long-distance hauling machine. Plus, there's just something satisfying about seeing that "Distance to Empty" readout hit four digits. It's a game changer, plain and simple.