Looking to put a thin aux tank in the bed of my truck but have never done one before.  Found some decent priced 37 and 50 gallon tanks at Northern Tools, Tractor Supply, etc.

Is there a kit that allows you to plumb it directly into the existing tank or filler tube that just stays open?  One that will allow the aux tank to free flow as your main tank level goes down?

The kits I have found all work using a petcock valve but I would like to avoid having to stop on the side of the interstate, get out and turning a valve on and I dont want to install a pump and run wires into the cab.

No, if it is above the main tank and has free flow into the main tank, the main tank will overflow.

You could design around that theoretically, but as soon as air is allowed into upper tank you'll overflow. Better to use a transfer pump.

A gravity feed is technically not allowed in a lot of states, but people do it.

Fwiw I’ve had em and tons of people I know have em, never heard of a person overflow the main tank.

Titan fuel tanks makes larger tanks you can install in the stock location . expensive, but you keep your bed and stock levels of safety.

It can be as simple or as complicated as you like.
Give some more details of what you want to do and why.
What pickup do you have?
Where are you located?
If you are in flyover country find a custom harvester and see what he is doing.

Went with the S&B 68 Gallons, no clearance issues.

Most folks do gravity fill and plumb it in to the filler neck with a check valve that shuts off when it's full.

Personally, my next truck will have a transfer tank in the bed, with a mechanical rotary pump and filler nozzle, but I have a tractor and other things to fill with it, so I wouldn't be using it only for aux fuel capacity for the truck.

I use a Transfer Flow brand tank. https://www.transferflow.com/

I believe they have some articles on their website talking about this question.

Why?  I always assumed those tanks were for OR equipment fills

I had a 90 gallon. Plumbed in with filler neck adapter. Ran it like that for 4 years. I had a ball valve on line to filler neck, just in case.

I have one, however mine is not "gravity fed", which is what you are describing.  That said, there is a kit (at NorthernTool.com) that splices into the filler hose.  There is a check valve that stops the flow from the aux tank when the fuel level rises to the check valve.  So, in theory, you could leave the petcock open all of the time.  However, if you forget the petcock is open and you open the truck's fuel filler cap with the transfer tank full of fuel, you are in for a bad time potentially spilling a lot of fuel.  Are we talking diesel here? - I don't think most of the aux and transfer tanks are technically legal for gasoline - I know mine isn't.

My setup:  2015 Ford F250 Diesel (with the puny 26 gallon OEM tank)

I have a 36 gallon transfer tank in the bed.  I fabricated a dip tube from hard fuel line and some adapters so it fit in the 2" (I believe) bung where the pump and handle assembly is supposed to go on the passenger side.  From there a hose connects down to a fuel filter/water separator then to a diesel inline fuel pump.  The pump is wired to a switch in the cabin.  The pump is spliced into the fuel filler hose with the  check valve kit from RDS (NorthernTool) to prevent me from overflowing the tank (in theory).  It isn't the fastest at moving fuel, but it does move it faster than it is used by the engine.  A quarter tank in about 10-15 minutes.  I may upgrade the pump at some point if I feel it is necessary.

There are also larger aftermarket tanks that replace the OEM tank.  No experience with these, just know they exist - https://titanfueltanks.com/

Let me take back my posts. If you had a reliable setup like a toilet has to let water in, that would work. Not sure that exists.

I would also include a auto shutoff valve so that when key is off valve would be closed. And also a manual shutoff in case that valve failed. But I'm paranoid

At least on the Ford short-bed - we are stuck with a puny 26 gallon tank thanks to the emissions junk under the truck.  When towing the camper, that means we have to stop way too often.

Been look at these also. Except for gasoline. I see one's that are fuel tanks and storage bins.

Do you recommend a installed pump or one that is mobile and comes in a case to store in tool bin.

Mine was not connected into my fuel system. Specifically so I could carry offroad when I needed to.

I had a 80 gallon I installed in my F250. Used a electric valve and a 13GPM pump. Installed 2 switches on the dash, 1 opened the valve and the other turned on the pump.

Could fill the road tank while still driving in just a few minutes.

Plumbed the line into the fill neck below the vent line, worked like a charm.

I have a Ram 3500 mega cab which as most know has the 6.7 Cummins.  My stock tank is 26 gallons and with my engine being "modified" I get 20 mpg empty.  With my 38 foot camper I get 10 mpg so I routinely make it 200 miles before I am looking to refill.  An aux tank gives me a lot more distance before I need to look for a fill-up.

I have seen the Titan tanks but they are North of $1k and that's a little steep for a plastic tank.

If you have links to the kits with the check valves, I would appreciate them.  I have heard the filler hose has a trapdoor that seals well enough to prevent an overflow situation but am interested in seeing any of these kits that come with check valves or something that shuts them off when the main tank is filled.

I am in rural flyover country with no types of inspections.

Looking at something like this, can get it in 37 or 50 gallon.  Doesn't take up a lot of room.

I've had a RDS 90 gallon aux fuel tank for my F-350 PSD since 2006.  I plumbed it with the RDS aux fuel conversion kit from Northern Tools.  https://www.northerntool.com/shop/tools/product_200315501_200315501

I have a letter from the USDOT stating that this method of refueling via aux fuel tanks is DOT legal*.  I installed my first aux fuel into a 1985 Nissan pickup back in 1986 or 1987 after an almost disastrous Nevada desert summer trip - running out of gas 50 miles from anywhere isn't advised.    At that time, gravity feed of gasoline was legal, it is not now.  I can provide the CFR regulation if anyone wants it.  [See CFR regs in reprinted USDOT letter below].  People who say that gravity feed of diesel is illegal, never seem to provide the list of states and or the specific regulation, even though I've asked for it numerous times.



Call RDS and find out where their nearest vendor is to you.  Call them, order the tank and get it via their normal weekly or bi-weekly shipment and save $100 or so on shipping like I did.

If I you add an aux fuel tank, add a fuel filter with water separator between it and the main tank.  I have four fuel filters on my PSD - two OEM, one aux fuel filter and one pre-pump fuel filter.

PM me an e-mail and I'll send you a very useful (IMO) paper on installing a diesel aux fuel tank onto your truck.    

Good luck.      



* Copy of text of USDOT from the USDOT Federal Motor Carrier Safety Administration, dated June 2, 2008:

Thank you for your April 21 letter concerning the Federal Motor Carrier Safety Regulations (FMCSRS) pertaining to fuel systems. Specifically, you requested clarification regarding whether a diesel-fueled vehicle supplying fuel from an auxiliary fuel tank, via gravity feed to the vehicle's original equipment manufacturer (OEM) fuel tank fill tube, is permitted under Section 393.65(d).

FMCSA is the Agency in the U.S. Department of Transportation (DOT) responsible for safety regulations concerning motor carrier operations. Our regulations include certain requirements for motor carrier safety management controls, driver qualifications, and commercial motor vehicle (CMV) equipment necessary for safe operations.

Section 393.65 of the FMCSRs provides general requirements that are applicable to systems for containing and supplying fuel for the operation of motor vehicles or for the operation of auxiliary equipment installed on, or used in connection with, motor vehicles. Specifically, Section 393.65(d) prohibits a fuel system from supplying fuel by gravity or siphon feed directly to the carburetor or injector. The attachment to your letter showed a "Diesel Install Kit for Auxiliary Diesel Fuel Tank" that is designed to connect an auxiliary fuel tank to a truck's OEM fuel tank fill tube. This device provides a consistent gravity feed of fuel, and automatically controls the fuel level going into the OEM tank must the use of a check valve so that it not overfill. You requested clarification regarding whether the use of such a device is permissible under Section 393.65(d).

Because the "Diesel Install Kit for Auxiliary Diesel Fuel Tank" supplies fuel from the fuel tank to the OEM fuel tank fill tube  and not directly to the carburetor Or
injector," the use of the subject device is permitted under Section 393.65(d). It is emphasized that all other requirements of Sections 393.65 and 393.67 must be satisfied.

@Merlin

IM sent, thanks for your help.

One thing to think about, 50 gal of diesel fuel weighs about 350lbs, plus the weight of the tank and accessories, you’re hauling around an extra 500lbs, which will have an effect on your fuel economy. Not sure how much it would be but if it were 1/2mpg that adds up over time. You also need to consider the extra weight when you’re towing if you are pushing GVWR limits.

ETA: Also, if you have an auxiliary tank plumbed into your trucks fuel system, you better not get caught with off road diesel fuel in it.

526kb inbound.  

Excellent point.  There are some real idiots out there trying to save pennies by risking thousands of $$ in fines.  I understand the usual fine is $10/gallon capacity.  In my case, that would be 38 + 90 gallons = almost $1300 bucks.  At $.50/gallon saved, I'd have to use +2500 gallons* to make up for it.  Not even close to worth it.

*If I did my math right.

I dont run farm diesel but more than once I have filled up at the local farm coop station and seen someone drive right up to the farm/OR fuel pump and fill their tank

Have you, with your own eyes, seen anyone check a tank?  They stopped doing it around here years ago, no one checks anymore.

I've never seen it, but my BIL has, even got his checked once or twice (he used to own a sod farm).

I understand that they don't check tanks any more; they use an exhaust analyzer, don't have to have access to the tank at all.  I don't use red fuel on my on-road vehicles, so don't look to me as an expert - other than "not gonna happen".

I have not. My father worked at a livestock auction a few years ago and they were checking trucks that were hauling cattle in. They caught someone he knows from what he said. I estimate you are 87% more likely to get checked if you have a transfer tank in the back of your truck.

Damnit!  Not the dreaded 87%  Once I get it installed, they can check if they want, I will be safe.

Fuck that! I’d demand a warrant. If that doesn’t work simply explain that as a sovereign citizen you are exempt from the illegal fuel taxes associated with on road fuel. Then simply ask if you’re being detained.

If you are going to pump gasoline you need a tank rated for it. As I said above, look at TRANSFER FLOW.

Another vote for Transfer Flo. Do it right. Love my 98gal one in my 2500 Silverado.

Over-priced ass holes.  When I was researching aux fuel tanks +15 years ago, I went to all the major tank manufacturers asking for help.  Had a very nice conversation with one of the TF/$$$$$ techs - until I asked about gravity feed of aux diesel tanks.  Holy Shit.  You'd have thought I asked him if I could rape his 3 year old daughter or some shit - and get paid for it.  Went completely nuts; fell apart like a cheap watch as my uncle used to say.

When I called RDS, we talked about gravity feed for a while, then I told the guy (I think it was their president or owner, don't recall) about TF's reaction.  He just sort of chuckled and then said something like:  "Yea, we get that a lot".  BTW:  It was this RDS guy who recommended the trick of using a local RDS vendor and having the tank shipped to them on their regular shipping schedule and saving $100 or so on shipping.  I thought that was pretty cool.  

TFs web site used to have a lot of language that basically tried to say that gravity feed was illegal.  It's not.  I fucking hate liars.  I especially hate over priced full of shit liars.


ETA: Yep, still full of shit:  https://www.transferflow.com/regulations

@TrippEd36 - Got a make/model or link for the 13GPM pump?

It was just a Northern Tool basic transfer pump, nothing special.
I think it was like $30 bucks, the 12V electric valve with 3/8 ports I ordered off Amazon.
I also had a fuel filter right off the valve.  Again it wasn1 I had picked up at Northern Tool.

If i were going in the bed I would go with a titantank polymer tank. They don't rust or corrode.

Under the bed same company.

I will be adding their big midship tank with a fleece lift pump to my duramax.

Those big electric fuel pumps sticking out of the bed get attention from dot and well theives is certain area

My first aux fuel tank was an Auxillary Fuel Systems (now defunct) plastic fuel tank that I installed in the mid-80's.  It was designed to be installed behind the rear axle above the spare tire.  I see Titan has the equivalent now - for about 5X the cost.  It was a simple gravity feed system into the main tank.  Easy, cheap and effective.  Of course the CFR changes since then mandating that gasoline fuel come out of the top of the tank put an end to that.

The plastic/polymer/whatever it was held up well the plus 15 years I had that truck, which I sold in 1999 or 2000.  I had thousands of miles of desert running in that truck.  Never a problem from the tank.  My record was ~550 miles of no pavement on one tank of fuel (15 OEM + 15 AFS tank) in eastern OR and NW NV.  I was on fumes when I coasted - literally - into the first gas station I found in Winnemucca, NV at midnight.  

I ran a 40 gal gravity feed system in my Powerstroke diesel for years. I took the fill hose for the factory tank and connected it to the out fitting of the in bed tank using reducing fittings. Then, and this is important, I connected the vent hose from the lower tank to the vent on the upper tank.
Worked like a champ. You can only fill the tanks using the upper tank fill spout since the lower was rerouted. The only issues I found were that I used a small diameter hose to connect the upper to the lower tank and it would take a few minutes to fill the lower tank after I filled the upper tank. Usually, I would get tired of waiting for it to gravity feed the bottom tank and would just drive off, knowing I had at least 40 gallons available. If I was going to do it again I would use at least a 3/4 inch hose. When taking a long trip I would fill both tanks and then have the freedom to pick my next fill up location and time. You are also hauling additional weight and generally you gauge will always read full.