Updated: 56 fuel tank capacity vs. gauge

[swatson999]

I hadn't really thought much about this until now, but I can't quite figure this out. The specs say the tank on a 56 is 17.5 gallons, and I'm assuming the guy who did the marvelous restoration of my new-to-me car put in the correct sized tank. But when the gauge gets down to nearly on E, it never takes more than around 12 gallons to fill it up.

Any ideas here? TIA!

[ward 57]

It could be as simple as bending the rod to the float so it has correct full range. I don't know about the '56 tanks but mine is easy to get to through a cover in the trunk.

[ICON 1956]

Ward is correct, I have a 56 , Lift up the trunk cover you will find the top of the sending unit. There are 5 screws to undo lift the sender up to expose the float attach to the rod. Since the gas tank shows "full" at 12 gallons you need to bend the rod upwards to allow for more fuel to register on the gauge.

[swatson999]

I must not have been clear...it shows "Full" when it actually *is* full (after topping off the tank), and moves off the F mark after driving for a bit (i.e., when a gallon or two has been used).

It shows "Empty" after burning only ~12 gallons, instead of 17, i.e., when there's ~5 gallons or so left in the 17.5 gallon tank.

[ICON 1956]

You might need a new gas sender gauge....Or your carburetor is rich burning too much gas.

[swatson999]

You might need a new gas sender gauge....Or your carburetor is rich burning too much gas.

How could the fuel mixture at the carb *possibly* impact the measurement of the fluid level in the tank?

My initial inclination is that it's not the sender (at least not the float) because the usual failure mode is for them to leak and *sink*, and I've never seen a liquid float sensor that can float above the surface of the liquid, suspended in air. :) That said, it *could* be, if perhaps the electrical characteristics of the sensor are wonky.

But that's what I'm looking for...help diagnosing the Fuel Gauge system.

I did read that new gauges have to be calibrated, so perhaps that could be it. Or the instrument VR? Or something else? Any advice is appreciated, of course...

[ICON 1956]

Here's a video on how to calibrate the fuel gauge.
https://www.youtube.com/watch?v=6DPHuemLdjk

[klogsdonjr]

I also find my gauge to read near empty when not fully empty. I replaced my sensing unit, old unit float was not punctured, new unit gave essentially same results even when I bent it.

I think the gauges, fuel, temp, and speedo just aren't as accurate as I'd like. I keep the fuel and odo readings in an app to track fuel efficiency. I know I can get 150 to 175 miles or so on a tank.

[ward 57]

At least you know you have enough reserve when reading empty like on the new vehicles that turn on a light when you are down to about 35 mile range.

[swatson999]

Yeah, at least I know I've got reserve. But it bugs the engineer in me to not have it *correct*. There's no reason for it to be so far off...and Ford engineers weren't dumb, so I presume it worked properly when it came off the factory line.

[klogsdonjr]

It does bug the engineer in me as well. Nothing like "unequal weights".

[Jim Yergin]

The replacement tank I installed in my '56 only holds 14 gallons. I verified that by draining the tank then refilling it one gallon at a time. Maybe when your car was restored a replacement tank like mine was installed.
Jim Yergin

[MarkR]

Jim, something is off. 55 and 56 birds were 17 gal., 57 is 20 gal.

[Rusty57]

Interesting thread going here. We seem to have a couple of potential issues to address.

The question of tank capacity is relevant for a key reason. If the replacement tank is not as deep as the original that means the it probably does not have the capacity of the original unless it has more width or length. The shallower depth means that the float on the sending unit cannot travel through the same range as it did in the original.

My experience with replacement/reproduction fuel tanks is that you really need to carefully and closely compare everything between the 2 tanks. In simple terms a 1/4" difference in height on a 30" square tank is a gallon of fuel. The last replacement fuel tank I installed that was supposed to be an "exact" replacement required us to modify the mounts in the vehicle as well as re-orient the filler.

My experience also is that the rated capacity of a tank is not the same as usable capacity. Unless the pickup is at the very lowest point of the tank you will not get all the fuel. The downside of that lowest location is that all the crud in the tank gets fed to the pump and carburetor. The same can be said of a drain plug. Unless you have the tank out of the vehicle and can freely rotate it in all directions the drain will not let all the fuel out.

The accuracy of these type sending units and gauges is always suspect. There is no chance of any linearity throughout the range for many reasons.
- The tank is not a prefect rectangle. The volume of fuel in the top 1" of the tank is not the same as the middle 1" or the bottom 1".
- The float arm moves in an arc, not a straight line. 1" of float movement at the full level is a more rotation than 1" of float travel at mid-range.
- The float arm moves a contact in an arc across a variable resistor ( rheostat). It is very difficult to design and especially manufacture a wound wire rheostat in an arc that gives the same difference for the same movement throughout the range.

There are some simple experiments you can perform to help you understand how these senders and gauges work. Take the sending unit out of the tank and connect it to the wire to the gauge. Connect a ground wire from the chassis to the sending unit. Turn the key on and watch how the gauge reacts as you move the float through the range of travel. Make some simple measurements of distance for the vertical range of float travel and compare that to the depth of the tank. Look at the position of the top of the float in relation to the sending unit mounting plate in the full position.

[Jim Yergin]

MarkR,
I am not disputing the capacity of the original tank. I am just pointing out that my replacement tank, while the outside dimensions are the same as my leaking original, the capacity is only 14 gallons. My suggestion is that before adjusting the sending unit the OP may want to first verify his tank's capacity by maybe draining the tank, refill with a gallon or two then drive to a nearby gas station and see how much more gas it will hold.
Jim Yergin