Exploring Alternative FJ40 Under-Bed Fuel Tank Options

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Jun 11, 2013
San Jose, CA
Greetings MUD! I've been lurking in the shadows for a while and it's now time to ask my first real question.

I'm midway through a series of upgrades on my 1971 FJ40, one of which is a 3FE conversion. To support the new motor, I had planned to go the simple route with the stock tank, an external in-line fuel pump, FJ62 FPR and be done with it. After hours spent staring at the truck, at the tank, and the usable space consumed by the tank, paired with the peace of mind of a hard barrier between myself and 18 gallons of fuel, I'm more and more considering going to an underbed tank.

I don't care for the rear ConFerr style tanks, not for any major reason, just a few minor ones: I don't like that it is visible from the rear, distracting from the stock aesthetic, I do not want to modify my fuel fill location, I want to keep fuel weight in front of the rear axle. The more obvious option of using a late model stock tank was considered, but I have not actively searched for one and was looking to squeeze out a little more capacity anyway. That leaves building from scratch. Skills/material to complete this are in hand.

My goal would be a minimum of 20 gallons, 2.7 cubic feet. I spent some time under the truck, and there does appear to be some good usable space in three main regions: 1: A large flat object, thin in height, from transfer output to differential, from frame rail to frame rail. Imagine a big square pancake. Fuel fill would be located in this shape, on passenger side, and (with some pipe work) would route to the stock location. 2: A longer and narrower shape 6" x 8" following the driver's frame rail forward as far as desired and rearward combining with the flat shape in #1, ending at the rear axle. 3: The largest volume would be the area opposite the rear drive shaft, from transfer case extending rearward to the rear axle, likely even passing over the driver's side axle tube on the outboard side, narrow missing a fully-compressed shock. This last area is where an in-tank fuel pump would be placed, more or less as centered in the tank mass as possible.

There are some anticipated downsides: Cruiser lean could be magnified, tight proximity to exhaust, relocation of muffler, difficulty in construction for baffles and general tank build

To 'fuel' the collective mind, here are some photos of 60/80-series tanks that I've pilfered from various threads on MUD, each embrace a little bit of what I'm looking for.

With those concerns on the table, has anyone gone down the path of a custom underbed tank, forward of the rear axle, with larger than stock capacity? Success, failure?

Thank you, Matt

Gas tank 60.jpg

1604395 60.jpg
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I did it! It was a success, but it was a PITA!
I couldn't fit a larger capacity on mine. Its a Wheeler, so I couldn't afford to have a tank hanging down in the rocks.
Used the stock filler location like you're figuring.
Building the tank is pretty easy provided your angles are spot on and edges match perfectly. I pressure tested it to 40 PSI (very time consuming) and sealed up all the little pinhole leaks.


Also, you'll probably want to figure for a sump or a sloped tank bottom to keep the fuel around the pickup location.
You are right about vehicle lean with 20 gallons on one side of the vehicle, the closer to the centerline (driveshaft) the better. How about 10 gallons on each side - - connected to empty at same time ???. I'm just thinking out loud.
I got ahold of a axle-forward tank from Conferr that was designed for a Toyota pickup truck. IIRC, even with pickup bed length, they could only get about 12 gallons, which is fine for an aux tank, but small for a main tank.

If you built a shallow tank for the rear to be used in conjunction with a forward tank, you'd have less lean and more than stock capacity. Run a transfer pump between them and call it good.:)


Mark A
Annother option would be the 28 gal Man-A-Free tank that replaces the tank under the front seat. It goes all the way across... But it's not under the floor.
Annother option would be the 28 gal Man-A-Free tank that replaces the tank under the front seat. It goes all the way across... But it's not under the floor.

I thought those were unobtanium, so I didn't mention it.

Dhondagod built his own version of the cross floor tank that included a deep, angled well in the center that would keep fuel in the pickup under almost any conditions. It was a lot of work, but it REALLY worked well off road.
They seem to be for sale now and then. Google had one in the clasifieds that doesn't seem to be sold.

Or, they could be fairly easily duplicated.
The only Man-A-Fre tank I find is the rear under the floor type?
My 2 pennies: my FJ didn't have a tank and custom roll cage wouldn't allow tank in original location. I looked at the same area - in front of rear axle.
I ended up installing Downey tank in rear. 2 things: I didn't like a fuel cell over my drive shaft- in case shaft comes loose at highway speed and it was cost prohibitive for a custom tank for this area.
The Downey tank snugs up really well and is very difficult to see from the rear of vehicle. Also, 160lbs of fuel is not much weight, relatively.
Keep us posted!
I like the idea of the tank under the two front seats. Or just make your stock on work. I am not afraid of having the tank in the cabin of the truck-its probably safer in some ways from damage. (The stock one is under a nice giant sheild which is way more protection than Ford ever provided. And if a tank is to explode(unlikely unless you totally neglect it) its gonna destroy the truck whether its in the cabin or not. COG is a bid deal thats the main reason I would keep it inside. 20 gallons is a lot of weight and it will affect how the truck handles offroad.
^X2, regarding main tank in cab^ The only problem I see with the main tank in the cab is the potential for fumes, but if you keep up with maintenance (hoses, charcoal can, etc.), that's not a problem either.

I actually placed my 22 gal aux tank behind the axle, like @Hentra and kept my main tank in place, in the cab.

The difference between my aux tank and @Hentra 's is, mine is not beveled in the rear... it is flat and thus shows more than the Downey fuel cell. My tank weighs about 40lbs, the skid play weights about 50lbs and 22 gallons of gas weighs about 139lbs - total added weight about 229lbs.

I can tell you, 44 handles even better with the empty tank in place... and even better with the tank full. I think the added weight causes the stiff rear spring to flex more.

I was concerned with departure angle, because my aux tank is not beveled in the rear

But, this past Monday, I drove the Morman Well Road in the Desert National Wildlife Refuge. This is the rougher road on the refuge and is very rough in places and smooth in others. It also has a washout area, about 38 miles into the 47 mile trip.

44 handled great on the whole trip... but, as I was coming out of the washout area, the last climb out of a wash was preceded by a dip and the hill was about 45* - if I couldn't climb out, it was a 30+ mile, 3 hour drive back to US95... I dropped to 1st in 4Hi and climbed right out. Never scraped or anything.

So, I'm completely satisfied with my aux tank behind the axle... I will have to watch for the same situation, with a large rock to hang on, as I enter the hill climb.

This is a great drive for anyone in the Vegas area!


I'm probably not going to make any more of the aluminum skid plates for the Downey tank because they cost so much (ties up so many dollars) and sells so slow (I only sell about 2-3 skid plates to every 10 tanks). BUT, with that said, the nylon full cell without skid plate still lab tested stronger than a 14 gauge steel tank- - - steel tank ripped, nylon tank only dented, then popped back out.
I pressure tested it to 40 PSI

Holy BEJESUS batman..... That's insane. Fuel tank leak tests are normally done at ~3PSI...

Think about how many square inches there are inside your tank, then figure the pressure you were putting on that!

I work for a boatbuilding company and we build plenty of fuel tanks. Certification requirement is 3PSI for marine fuel tanks - and they take much more of a beating than and vehicle tank would ever take. (Ok, maybe similar to Baja racers?)

If you want more info on testing, google CFR46 183.580 (page 100). or ABYC H24
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Hey dude...
I'm a rookie! Figured if it holds at that pressure, it'll be good.
Holy BEJESUS batman..... That's insane. Fuel tank leak tests are normally done at ~3PSI...

I worked for a plastic fuel tank manufacturer....using Helium, it is only 1.5 psi.

Putting over 5 psi in a square plastic tank makes it start to turn into a ball.
Hahaha, no worries - wasn't trying to be like the bezel police or anything, just blew my mind at that pressure!

Hey Flipper1938, you can balloon 500 galon aluminum tanks too... Just leave a little acetone in a rag inside the tank and send a welder in to finish up... ;cD

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