Gasoline Transfer Tank.

delete - not relevant.

The electric fuel pump I got from NAPA easily pumps the fuel from the bottom of the aux via the tank pick up tube ( dip tube).

Matter of fact, once the process is started, mine would siphon slowly on its own from the aux due to the different elevations.

To stop that i also installed a normally closed 12 volt fuel shut off valve in the line.

Sport45 wrote:
The law essentially says a gasoline tank cannot withdraw fuel from a connection below the fuel level.

49 CFR 393.67 (c) wrote:
5. Fuel withdrawal fittings. Except for diesel fuel tanks, the fittings through which fuel is withdrawn from a fuel tank must be located above the normal level of fuel in the tank when the tank is full.

I take this to mean an in-bed auxiliary tank with top connections can pump or siphon to a vehicle main tank without issue.

If the 2 tanks are connected together via a gravity feed tube, then the normal fuel level is the level in the aux tank, far above the top of the truck's main tank.

The truck would have to draw directly from the aux tank, not from it's main tank (and thus the main tank becomes the aux tank), for the law to be (partially) satisfied. There's still the issue of the connection at the bottom of the in-bed tank.

Pumping the gas out of the main tank via a pump in the in-bed tank would likely not be possible, because the pump in the in-bed tank likely would not be able to draw up the fuel from the truck's main tank, through the tube connecting them together.

That explains the gravity feed issue.

As far as pumping from the top of the aux tank, that is legal, but not sure if it is legal while driving down the road. You can have an aux gas tank with a pump nozzle on it that you pump into the fill neck of the main tank, as needed, when pulled off the road. That way the 2 tanks are not connected together in operation.

Or, you can add an additional gas tank under the truck at a similar level as the main tank and connect it to the main tank with a switching valve. This was common on older trucks (1960's - 90's) that came from the factory with dual tanks.

Deleted due to content not applicable to tanks under discussion.

The law essentially says a gasoline tank cannot withdraw fuel from a connection below the fuel level.

49 CFR 393.67 (c) wrote:
5. Fuel withdrawal fittings. Except for diesel fuel tanks, the fittings through which fuel is withdrawn from a fuel tank must be located above the normal level of fuel in the tank when the tank is full.

I take this to mean an in-bed auxiliary tank with top connections can pump or siphon to a vehicle main tank without issue.

SoCalDesertRider wrote:

Sport45 wrote:

SoCalDesertRider wrote:
The law is you cannot have a gasoline aux tank that gravity feeds into another gas tank (ie: feeding into the truck's main factory gas tank). It's legal to do this with diesel, NOT with gasoline.

Is that the law? I thought you weren't allowed to gravity feed to the engine. But I've been wrong before.

I have no idea if there is a law about gravity feeding to the engine in an on-highway vehicle. I don't know why anyone would be trying to do that on a pickup. The gas tank would have to be on top of the hood...

There are lots of stationary use engines that use gravity feed tanks instead of a fuel pump, such as generators, lawn mowers, air compressors, water pumps, etc.

I am thinking Sport was talking about gravity feeding the GAS TANK with a in bed aux gas tank.. Not directly gravity feeding the engine.

Gas has a higher expansion/contraction ratio than diesel..

Not to mention gas is more volatile and evaporates compared to diesel which is barely volatile and does not evaporate (spill some diesel and that puddle will be there pretty much forever).

Due to the high expansion/contraction ratio and fast evaporation it makes for a less than ideal fluid for gravity from one tank to another on a vehicle.

Not to mention, overfilling your vehicles tank with gas WILL result in the "evap" system taking on raw liquid gas instead of fumes, this WILL result in damage to the evap system ($$$).

The Evap system has a vent on the top of the gas tank, that vent is routed to the evap canister. This canister collects the gas vapors from the tank and eventually releases those vapors into the engine..

If one was to put a gravity fuel tank above the vehicles tank the evap vent would have to be relocated to the top of the highest tank..

Then there is the little matter of the vehicles fuel cap, one would have to lock it to ensure the cap is not removed while the top tank is full.. would kind of stink to lose 20 or more gallons of gas when someone who doesn't realize that the top tank is plumbed in and attempts to remove the fuel cap.. :E

Sport45 wrote:

SoCalDesertRider wrote:
The law is you cannot have a gasoline aux tank that gravity feeds into another gas tank (ie: feeding into the truck's main factory gas tank). It's legal to do this with diesel, NOT with gasoline.

Is that the law? I thought you weren't allowed to gravity feed to the engine. But I've been wrong before.

I have no idea if there is a law about gravity feeding to the engine in an on-highway vehicle. I don't know why anyone would be trying to do that on a pickup. The gas tank would have to be on top of the hood...

There are lots of stationary use engines that use gravity feed tanks instead of a fuel pump, such as generators, lawn mowers, air compressors, water pumps, etc.

SoCalDesertRider wrote:
The law is you cannot have a gasoline aux tank that gravity feeds into another gas tank (ie: feeding into the truck's main factory gas tank). It's legal to do this with diesel, NOT with gasoline.

Is that the law? I thought you weren't allowed to gravity feed to the engine. But I've been wrong before.

I had a 100 gal tank in the back of my 98 chevy pumped out of the top of the tank into a t in the filler neck connected the vents together so gas fumes did not escape into the air only thing I could see wrong with this setup was my evap system was only rated for 120 gallons I had a 130 gal capacity

I don't know about a limit on how many gallons of gas you can carry in the bed. If there is, nobody's enforcing it.

At the off highway vehicle riding areas in California, most folks carry several 5 gallon jugs of gas for their bikes, buggies, etc, usually in the bed of the truck, or on the trailer. Also, toyhauler trailers often have built in gas tanks with a pump, for filling the toys, which are often 20 gallons or more.

On my welding truck, the gas self-generating welding machine has a 12 gal tank, the gas air compressor has a 2 gal tank and I carry usually one, or sometimes two, 5 gal cans of extra gas. DOT and OSHA haven't complained about anything yet.