What's the absolute max useable fuel in a Duster?

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cruiser

cruiser

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Hi All: I'm trying to figure out the maximum range of my 1974 Duster. My car is bone stock with the 225 six and Holley one barrel carb. Entire fuel system, including the new 16 gallon tank and rebuilt sending unit, is all stock. I get about 14.5 to 15.0 MPG at 70 MPH in highway cruising. I've also installed and calibrated a Meter Match electronic fuel gauge calibration unit, which seems to be working well. The advertised capacity of the fuel tank is 16 gallons. When I'm at exactly half full on the gas gauge, the car will take 10.5 gallons of gas to fill it. Half of 16 is 8, but I'm able to put 10.5 gallons in. I'm thinking that the extra 2.5 gallons goes to fill up the filler neck. That's why I have to go about 30 miles before the gas gauge needle starts to come off "F". So does that mean that 8 more gallons of useable fuel is in the tank when the gauge shows half full? If that's the case, this means that I can go another 120 miles (15 x 8) until the engine quits, right? Is there unusable fuel in the bottom of the tank that the pickup cannot suck up and send to the engine? When the refueling hose auto shuts off when the nozzle senses the filler neck is full, is the tank really completely full, or is there a trapped air bubble at the top of the tank? I'm thinking about filling the car completely full, then driving until the engine quits to see exactly how far it'll really go on a single tank. I'd have to carry a spare container of fuel to do this, of course. So whaddya think, guys?
 
Fill it up, put a gas can in the trunk and drive her till she dies. Probably be about a gallon left in the tank but pickup height can vary from different sending units.
 
I'd say your meter match may be slightly off. When you get down below 1/4 tank throw a 5 gallon can in the trunk and run it empty. Then you will know for sure.
I've done this with every used vehicle I've owned.
Give your OCD a vacation! :rolleyes::)
 
Here's the latest: Today I ran the engine until it quit due to fuel starvation. When it quit, the fuel gauge was showing 3/8 full. I re-calibrated the empty tank to read "E" on my gauge by recalibrating my Meter Match unit. I was then able to fill the tank completely with 15.1 gallons of gas. This tells me that my brand new 16 gallon fuel tank doesn't really hold 16 gallons. There must be an unfillable air pocket at the top of the tank. Also, the filler neck fills up as well. So including the filler neck, my fuel system holds a maximum of 15.1 gallons. The engine quit after running 227 miles. Not very good range.
 
MoparMike1974 said:
Fill it up, put a gas can in the trunk and drive her till she dies. Probably be about a gallon left in the tank but pickup height can vary from different sending units.
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Yup. This here ^^^^ is the only way to know for sure.
 
RustyRatRod said:
Yup. This here ^^^^ is the only way to know for sure.
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Rusty - I'm a bit disappointed that I can only get 15.1 gallons of gas into my new 16 gallon tank and filler neck. I'm pretty certain that the tank was completely empty when I filled it. I removed the filler cap and sloshed the back end of the car back and forth with my ear to the open filler neck. I couldn't hear any fuel sloshing around in it. It seemed completely dry.
 
cruiser said:
Rusty - I'm a bit disappointed that I can only get 15.1 gallons of gas into my new 16 gallon tank and filler neck. I'm pretty certain that the tank was completely empty when I filled it. I removed the filler cap and sloshed the back end of the car back and forth with my ear to the open filler neck. I couldn't hear any fuel sloshing around in it. It seemed completely dry.
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I can understand that.....and completely relate. It's either one of two things. Either the factory "rounded up" (quite inaccurately) the original tank's capacity trying to compete with the competition, OR the new tank had corners cut when it was designed and made. What brand tank is it?
 
RustyRatRod said:
I can understand that.....and completely relate. It's either one of two things. Either the factory "rounded up" (quite inaccurately) the original tank's capacity trying to compete with the competition, OR the new tank had corners cut when it was designed and made. What brand tank is it?
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I don't recall the brand, other than to say that the replacement tank looked identical to the factory original tank when I had them side by side on my driveway.
 
cruiser said:
I don't recall the brand, other than to say that the replacement tank looked identical to the factory original tank when I had them side by side on my driveway.
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Then my suspicion would be that the factory just "called it" a gallon tank. Wouldn't be the first time the factory fudge numbers.
 
The bottom of the factory tank is large enough in area that I doubt you'd hear any sloshing at all with 1-2 gallons still in it.

It's physically impossible for the fuel pump to access 100% of the fuel in the tank. Even with the pickup touching the bottom, the thickness of the pickup has to be greater than zero. The fuel pump can't draw fuel if there is air present at the pickup. With about 1-2 gallons in the system, air will start to be admitted and pressure to the carb will drop.

The tank is over 30" wide and the flat at the bottom is probably 20" long. With a half inch of fuel down there, that's 1.25 gallon with 1/2" of fuel in there. Plus the lines, which can hold 1/8-1/4 gallon depending on if you have a return line. The filler neck holds maybe .75-1 gallon (assuming 2.5" diameter and 36" long).

If you have a total of 17.125 gallons including lines and the filler, that means you have about 2 gallons left in the tank plus whatever may have evaporated from the bowl and charcoal canister or other vents while running.

Remember, at operating temperature the gas tank is well above the evaporation temp for gasoline - losing 1/2-1 gallon over time wouldn't be unexpected IMO, especially if you don't have a functioning charcoal canister. Non-stop range may be longer as a result, since less fuel should be lost vs when the car spends time parked (in the summer, my vented gas cans for my lawn equipment will lose a gallon in about 2 wks).

With all this in mind, I'd say your usable fuel when empty is right about where it should be for a 16 gallon tank.
 
I cannot speak to your 16-gallon tank, but my 67 Barracuda has an 18-gallon tank, and some things may be similar. The pickup point (with the suction filter) is “towards” the bottom of the tank, and the float the goes down to about the same level. That’s fine, but most people do not know that there is about 3.5 gallons below that point.


The float is at its lowest level and the pickup tube is just above the level of the gas. So, you’re out of gas, but you have 3.5 gallons left. I know this for a fact. I tested it.
 
cruiser said:
Rusty - I'm a bit disappointed that I can only get 15.1 gallons of gas into my new 16 gallon tank and filler neck. I'm pretty certain that the tank was completely empty when I filled it. I removed the filler cap and sloshed the back end of the car back and forth with my ear to the open filler neck. I couldn't hear any fuel sloshing around in it. It seemed completely dry.
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Well yeah, but did you get a lighter to check if you could see any fuel in the tank? :lol:
Seriously though, I’ve always wondered why Ma didn’t put in bigger tanks. I know you just installed one, but there are 18 gallon tanks you can order for 2nd gen A bodies.
 
g413 said:
I cannot speak to your 16-gallon tank, but my 67 Barracuda has an 18-gallon tank, and some things may be similar. The pickup point (with the suction filter) is “towards” the bottom of the tank, and the float the goes down to about the same level. That’s fine, but most people do not know that there is about 3.5 gallons below that point.


The float is at its lowest level and the pickup tube is just above the level of the gas. So, you’re out of gas, but you have 3.5 gallons left. I know this for a fact. I tested it.
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Do you know if there’s a similar amount a fuel below the pickup in 16 gallon tanks?
 
I do not know the characteristics of the of 16-gallon tank.
 
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