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#1
Old 08-08-2005, 12:37 AM
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Purpose of dual gas tanks?

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?
#2
Old 08-08-2005, 12:47 AM
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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?
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#3
Old 08-08-2005, 02:21 AM
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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.
#4
Old 08-08-2005, 02:29 AM
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Quote:
Originally Posted by Sunspace
Were they switchable? If so, could you have a reserve tank and a main tank, like on a motorcycle?
That's always been my take. I think it's supposed to help you keep from running out of gas.
#5
Old 08-08-2005, 02:47 AM
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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.
#6
Old 08-08-2005, 03:36 AM
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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.
#7
Old 08-08-2005, 03:55 AM
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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.
#8
Old 08-08-2005, 04:10 AM
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Quote:
Originally Posted by ParentalAdvisory
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?
#9
Old 08-08-2005, 04:16 AM
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Quote:
Originally Posted by [email protected]
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.
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.
#10
Old 08-08-2005, 05:05 AM
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Quote:
Originally Posted by Sunspace
Were they switchable? If so, could you have a reserve tank and a main tank, like on a motorcycle?
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.
#11
Old 08-08-2005, 06:26 AM
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Quote:
Originally Posted by Eonwe
Yeah, but why not one huge gas tank?
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.
#12
Old 08-08-2005, 07:52 AM
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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.
#13
Old 08-08-2005, 08:32 AM
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Quote:
Originally Posted by Pushkin
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.
That's still the case. Flipping to reserve just means switching to a fuel line that's at the bottom of the tank.
#14
Old 08-08-2005, 09:27 AM
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Quote:
Originally Posted by Eonwe
Yeah, but why not one huge gas 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.
#15
Old 08-08-2005, 10:05 AM
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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.
#16
Old 09-29-2015, 10:29 PM
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Fuel

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)

Last edited by ss457; 09-29-2015 at 10:31 PM.
#17
Old 09-29-2015, 11:49 PM
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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.
#18
Old 09-30-2015, 09:50 AM
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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.

Last edited by bob++; 09-30-2015 at 09:50 AM.
#19
Old 09-30-2015, 10:45 AM
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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.
#20
Old 09-30-2015, 11:07 AM
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Quote:
Originally Posted by Hari Seldon View Post
... 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.

Last edited by LSLGuy; 09-30-2015 at 11:08 AM.
#21
Old 09-30-2015, 11:57 AM
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Quote:
Originally Posted by LSLGuy View Post
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.
Actually, it all normally happens in the fuel valve itself. The valve has three settings, ON, OFF, RES. The valve is at the low point of the tank with a tube 1-2 inches long pointing upward. ON draws from the tip of the tube. REServe draws from the base of the tube. OFF is, off.

Sample
#22
Old 09-30-2015, 11:58 AM
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I have an old F150 with an extended cab and an 8 foot bed. It has two tanks, a front tank and a rear tank. One is not a reserve. they are both 19 gallons each.

There is one fuel gauge, and a switch underneath it on the dash to select the front or rear tank. Whichever tank is selected is not only the one that is supplying fuel to the engine but it's also the one that is connected to the fuel gauge on the dash. So if you want to see how much fuel is in the other tank you have to actually switch over to it.

The reason for two fuel tanks is kinda obvious. If you look under the truck, there isn't room anywhere for one huge tank. There's room at the back of the truck for one smaller tank, and room on both sides for a smaller tank, but the drive shaft prevents you from having enough room to put a really big tank right under the middle of the truck. So they put one in the back and one on one side, so that you can fill them up from the same side of the truck. If they balanced it out and put one on each side then you'd have to fill the other tank from the other side of the truck.
#23
Old 09-30-2015, 12:02 PM
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Quote:
Originally Posted by Pushkin View Post
Do bikes have two separate tanks?
...And just to round things out, the old Harleys and Indians had twin tanks, but these were separate tanks that were connected by tubing. They were probably made as separate tanks so that they could fit around the frame. The gas petcocks were still set up to draw from a lower level when switched to reserve.
#24
Old 09-30-2015, 12:47 PM
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Quote:
Originally Posted by ss457 View Post
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)
I don't know where you're getting your info, but I think it is incorrect.

Regulations for fuel hauling you are citing are for bulk carriage, not actual vehicle fuel tanks. Even so, the maximum quantity haul-able without being placarded is 119 gallons IIRC.

Large semi trucks routinely have two 100 gallon or more fuel tanks, and they aren't placarded. The reason pickups have two tanks is because of 1) cost and 2) space. You can fit two smaller tanks in a pickup a lot easier than on big one. Also, having two tanks means for normal driving you don't have to fill both, saving cost and weight.

Last edited by Just Asking Questions; 09-30-2015 at 12:48 PM.
#25
Old 09-30-2015, 01:24 PM
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Like a Pontiac from Arizona, another thread has risen from the ashes.
#26
Old 09-30-2015, 02:28 PM
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Quote:
Originally Posted by engineer_comp_geek View Post
If they balanced it out and put one on each side then you'd have to fill the other tank from the other side of the truck.
I had an '83 GMC pickup like that and it was a big pain in the ass.
#27
Old 09-30-2015, 02:29 PM
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Originally Posted by herman_and_bill View Post
Like a Pontiac from Arizona, another thread has risen from the ashes.
Pontiacs come/came from Detroit.

#28
Old 09-30-2015, 02:37 PM
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2 Tanks is also more fuel per fill up meaning fewer fill ups meaning less time spent getting gas, even if its a pain in the ass to fill them both up its still easier than doubling your trips to the station.

If you have ever commuted on a motorcycle you probably know what I mean. Mine pulled 50 or so to the gallon but had a 2 gallon tank. Meaning triple the fill ups compared to my car which got 300 or so per tank.
#29
Old 09-30-2015, 02:47 PM
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Quote:
Originally Posted by herman_and_bill View Post
Like a Pontiac from Arizona, another thread has risen from the ashes.
Well, as long as it's already up and moving around, I'll nudge it a little more!

I have a fairly normal 2 door CAR with two fuel tanks. 1983 BMW 320i. It has two fuel tanks because it was the best way to creatively fit an acceptable amount of fuel into a space that was split by the drive shaft. No reserve function at all, the tanks are connected via a large hose, so fuel pickup is just in one tank. The "added-on" tank is filled or emptied just by it's connection to the other tank.

So, one case here of it was definitely done just for the packaging.
#30
Old 09-30-2015, 03:17 PM
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They still install a 2nd fuel tank

Mostly its for range.

Here in Europe a few people have "switchable" duel fuel tanks, to burn cheaper farm diesel from one tank while "working on the farm" and regular diesel when on the public road.

Last edited by Doughbag; 09-30-2015 at 03:20 PM.
#31
Old 09-30-2015, 09:57 PM
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Quote:
Originally Posted by Kimballkid View Post
I had an '83 GMC pickup like that and it was a big pain in the ass.
Park in between pumps and fill from both sides at the same time. We did that a lot in our old Chevy.
#32
Old 09-30-2015, 11:02 PM
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Speaking of old Chevy pickup gas tanks, and lest we forget 2,000 people were burned alive for the sake of $2.20 a truck:

Quote:
Based on data from the Fatality Analysis Reporting System (formerly known as the Fatal Accident Reporting System), over 2,000 people were killed in fire crashes involving these trucks from 1973 through 2009. . . .

. . . At the heart of GM's resistance to improving the safety of its fuel systems was a cost benefit analysis done by Edward Ivey which concluded that it was not cost effective for GM to spend more than $2.20 per vehicle to prevent a fire death. (Attachment N.) When deposed about his cost benefit analysis, Mr. Ivey was asked whether he could identify a more hazardous location for the fuel tank on a GM pickup than outside the frame. Mr. Ivey responded, "Well yes...You could put in on the front bumper." (Video: "Ed Ivey Deposition," Attachment O is a copy of this portion of the deposition.) GM was able to hide all this evidence and much more from the public until the early 1990's when leaks in GM's secrecy dam started and the Center for Auto Safety began to focus in on the defect.
#33
Old 10-01-2015, 11:18 AM
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Some people use their pick-up for around town driving only. They can fill up with gas anywhere.

Other people may use the pick-up for a camper or towing a trailer - travel long distances. Some places like western Texas may not have gas stations for a long distance, so travelers like a truck with more fuel capacity.
#34
Old 10-01-2015, 11:54 AM
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Our '61 VW Bug had a reserve like a motorcycle, with a foot operated valve forward of the shifter. I could never remember what position was what. AFAIK, once fuel gauges were standard equipment, that feature was eliminated. Same for motorcycles- my '04 Honda has a gas gauge, no reserve.
#35
Old 10-01-2015, 02:04 PM
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Quote:
Originally Posted by Pinmin View Post
Park in between pumps and fill from both sides at the same time. We did that a lot in our old Chevy.
I haven't had that pickup in years so it's no longer an issue. I've seen what you describe done before and I think it's a dick move.
#36
Old 10-01-2015, 02:48 PM
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I have the same truck with dual tanks, I think they are twenty gallone each. These trucks would get pretty poor gas mileage with a camper on the back. Maybe less than 200 miles. This may not be enough to go to a remote location and then drive around for a week or two while you are there. There are times when 40 gallons in not really enough.
#37
Old 10-01-2015, 02:58 PM
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Quote:
Originally Posted by Kenm View Post
Speaking of old Chevy pickup gas tanks, and lest we forget 2,000 people were burned alive for the sake of $2.20 a truck:
Probably more like 150. The Chevy C/K saddle tanks thing is one of the more flagrant examples of an automotive moral panic. (See also: Corvair, Pinto, Toyota acceleration, etc.) The Dateline segment that set it off was later shown to be deliberately faked, and independent studies didn't come even close to agreeing with CAS' ridiculously high fatality numbers. https://en.wikipedia.org/wiki/Chevro...nk_controversy

The saddle tank design was not ideal, but the effect on the truck's overall safety record was statistically insignificant. They were just as safe (or not, more realistically) as the equivalent full size pickups made by Ford and Dodge. It was also WAY safer than previous generations of trucks that had the tanks in the cab behind the seats.

Quote:
Originally Posted by boytyperanma View Post
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.
The requirement of two in-tank fuel pumps is probably what did away with the dual tank setups. With a carburetor and manual fuel pump all you need is the cheapo selector valve and a bit of extra fuel line. With EFI you need the two fuel pumps and the associated wiring. Plus in-tank fuel pumps don't like being run out of fuel, which you'd be doing all the time on the primary tank. When fuel injection began to be offered in trucks in the late 80's, you could get an FI-ed truck with two tanks and two fuel pumps but it was only a stopgap measure until they eliminated the dual tank setup in the next redesign.
#38
Old 10-01-2015, 04:38 PM
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Quote:
Originally Posted by GreasyJack View Post
Probably more like 150. The Chevy C/K saddle tanks thing is one of the more flagrant examples of an automotive moral panic. (See also: Corvair, Pinto, Toyota acceleration, etc.) The Dateline segment that set it off was later shown to be deliberately faked, and independent studies didn't come even close to agreeing with CAS' ridiculously high fatality numbers. https://en.wikipedia.org/wiki/Chevro...nk_controversy
Thanks for that. By this time I should expect faked reports.
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