I used to have a '91 Ford F150. It was my first car, so I didn’t have much say in what I got (in fact, I didn’t even want a truck, but that’s beside the point). One thing that struck me as odd was the fact that it had two gas tanks. That was a luxury I never used, or even thought of using. Anyway, eventually I got rid of the truck and moved on. Then tonight I saw one that was nearly identical to my old one. And again, I noticed the two gas tanks. It got me to wondering: What, exactly is the point of having two?
Were they switchable? If so, could you have a reserve tank and a main tank, like on a motorcycle?
Or maybe there couldn’t squeeze the desired tank volume into just one space under the truck?
Not sure about the F150’s, but I heard that the reason the Humvee’s had two gas tanks was so that if one got shot out or gashed open while going through rough terrain or whatnot, it could still make it home. Maybe the utility trucks like the F150 have this incase of the same thing (well, the second thing, anyways)? Or maybe it’s just something they imitate off the military vehicle for the cool factor.
That’s always been my take. I think it’s supposed to help you keep from running out of gas.
It also gives you the option of buying a lot of fuel when the price happen to be lower.
In large over-the-road trucks, they are operating on a very tight margin. Saving even a few cents per gallon on fuel may be quite important to the bottom line.
All I know is that I have a friend who had an Econoline which had two tanks, which you could switch between using a switch.
One purpose is range. With a single tank, maybe you can go 350 miles (estimate), with two, you get to go 700 miles. Really good for rural country driving.
Yeah, but why not one huge gas tank?
On the other hand one presumes that the weight of the extra fuel supply carried would decrease the payload, and the gas mileage. I realize gasoline weighs less than water, but still it weighs something.
Do bikes have two separate tanks? I only ask because my Dad’s old Royal Enfield (a 1960s vintage) has a smaller section of the tank at the bottom that can only be used if you push in a small valve on the fuel line. As there is no fuel guage on the bike, when you find yourself pushing in that valve, you know you have a certain amount of fuel left in the bike (hopefully enough to reach a petrol station) But as far as I’m aware, the reserve was only a smaller portion of the tank, not a separate tank altogether.
Aside from what’s been mentioned, weight could be a factor.
Gasoline is about 6lbs per gallon. So… a 10 gallon tank weighs 60 lbs which is no big deal. A 100 gallon tank weighs 600 lbs - that’s going to require sturdier construction. Would also imagine the sloshing when it’s less than full would be impressive. Maybe it’s easier/cheaper to supply multiple smaller tanks than one big one? Might also have something to do with sloshing.
Probably because Ford decided to supply X amount of fuel for this truck. If X was all in one tank, it would be difficult to fit it all into the small* areas bounded by: Axle, Rear Bumper, Driveshaft, and rear of the cab.
I moved from a Ford PU to a Chevy on my last purchase. To get the amount of gas reserves I wanted (35 gallons), I had to order a long bed. The short bed only held something like 28 gallons, presumably they’re unable to fit a large single tank into the available spaces. Trucks get abysmal gas mileage (mine gets about 12 city / 14 hiway) and you need to carry a lot of fuel to get decent range. My Ford had two tanks, but for some reason Chevy prefers only one.
*small relative to the amount of fuel.
That’s still the case. Flipping to reserve just means switching to a fuel line that’s at the bottom of the tank.
Another vote for size. Fitting one large tank would be hard to manuever around the chasiss of the truck for it to not scrape the road. 
Two skinny narrow tanks inside the rear edges of the truck bed is better for design. And like mentioned, if it’s going to be a large single tank, you’ll need longer (bigger?) truck, to hold it.
I used to participate in the building of fuel tanks for the F150 (previous generation) and the Super Duty. Similiar trucks, vasty different tanks. For the F150, there was a 4x4 model, short wheel base model, and long wheel base model, and they were sized in that order from smallest to largest. They were oblong and looked like they’d fit lengthwise inside the frame. The Super Duty (I guess that’s probably the 350) had an absolutely huge, squared-looking tank. I remember seeing it sitting in its chassis taking up every bit of room. Because of that and because of being a huge tank, I don’t think there was a dual tank option for the Super Duty. As for the F150, I suppose there could still have been dual tanks – they were symmetrical so possibly they could. In fact, because the 4x4 tank was so small, I’d almost always expect that it was dual tank.
My current Ford Truck has only a single tank, and I don’t know anything else about dual tanks at all. Are there two filler areas? We didn’t make and F150 tanks without filler necks or anything else special, so I’d have to imaging that yeah, there would be two filler caps.
The current, brand new Mustang has an interesting dual-like single tank that’s specifically done for packaging. By itself it looks like a saddlebag, and there are two halves that connect at the top above the drive shaft. There’s an internal fuel pump that ensures that gasoline is pumped into and sucked out of both sides evenly.
The difference in tanks is the regulations at the time on how much fuel you can haul in a standard vehicle. Two tanks gave more capacity with laws at the time. The bigger trucks only had one big tank if they are diesels because they fall under different regulations than gasoline. A tank can only hold 37 gallons of gasoline without having to be placard and require a cdl driver with todays laws. I.e a 37 gallon transfer tank can be rated for gasoline and anything bigger will say it is for fuel oils (diesel)
Twin tanks also brings in some redundancy which can be important in isolated areas. Many of the trucks also have two fuel pumps. So if a tank leaks or a fuel pump fails you still have options to work with.
Total volume is a factor in vehicle handling, you could have one large tank but you’d need more baffling as you can’t have 50 gallons of fuel sloshing around full force.
Manufacturing. If you use two tanks it might mean you only have to have one tank design for your trucks. Not all trucks have room for two tanks. Trucks that have the space can use two of the same rather than making one large and one small tank.
Many heavy trucks in Europe have two fuel tanks. There are two because of the limited space on a short EU style tractor unit, and the need for balance. 600 litres of fuel weighs around half a tonne. The reason they want a large capacity is because fuel from their own supply is much cheaper than that bought on the road. There are also differences in price between countries.
Anybody here remember the first VWs to come to the US? They had no fuel gauge, but they did have a reserve tank with a foot lever to kick the reserve tank in. When you felt the engine sputtering, you kicked the lever and got an extra gallon (about 30 miles) to run on. This happened to me once when I was on the PA pike around midnight and all the gas stations were closed. We (it was a friend’s car) got off the pike and, fortunately, found an all-night truck depot and convinced the operator (who wasn’t supposed to be selling fuel) to let us have a couple gallons to get home. I assume the buck we gave him (gas was a quarter a gallon in those days) went right into his pocket.
But could this whole insane setup, reserve tank and lever been cheaper to build than adding a fuel gauge? I don’t see how.
There’s not really a reserve “tank”. What there is is a single gas tank which has two fuel lines connected to it. The regular line connects about a half inch above the bottom. The reserve line connects at the bottom.
When the fuel level gets down to 1/2", the regular line starts sucking air & the engine sputters. So you open the reserve valve and Ta Da!! you have another 1/2" of fuel left in the same tank to burn.
This is how about 95% of modern motorcycles do it. A couple inches of extra tubing & a T-valve is all that’s needed. Back when electrical stuff was finicky and expensive this solution was far simpler, more reliable, and much cheaper than a fuel gauge.