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Old 10-01-2020, 04:39 PM   #1
19Fordy
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Default Gas Tank Fuel Gauge reading question

If you drain your gas tank on a car like a 40-48 Ford and fill it with water, will the fuel gauge give the same readings as when tank was filled with gas.

I ask this question because I am wondering if the difference in the density of gasolene and water will effect the float level and hence, the fuel gauge reading.

I would like to do this to check the accuracy of my fuel gauge and calibrate my gas tank sending unit.
Density of water is 1. Density of gasolene is .71 to .77 according to Google

Thanks.
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Old 10-01-2020, 06:57 PM   #2
terry
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Default Re: Gas Tank Fuel Gauge reading question

Given the density figures you gave, I'm thinking your gauge would read a little low. The oil that gas comes from floats on top of water. Just guessing. Terry
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Old 10-01-2020, 09:12 PM   #3
Zeke3
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Default Re: Gas Tank Fuel Gauge reading question

I think the float will sit a little bit higher in water than in gasoline because it will not have to displace as much water to reach the point of equilibrium, so your gauge readings would be slightly higher with using water. I am guessing that the difference in the float depth is probably pretty small compared to the overall range of the float travel of the sending unit from full to empty.

Michael Driskell posted his gauge calibration process for a resistance stlye sending unit in the recent discussion about replacing the fuel tank on a 1947 car, post #14 in the attached link.

https://www.fordbarn.com/forum/showthread.php?t=287943

Last edited by Zeke3; 10-03-2020 at 09:34 AM. Reason: Added link.
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Old 10-01-2020, 09:18 PM   #4
JSeery
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Default Re: Gas Tank Fuel Gauge reading question

Well, it is going to be different, but the question is, is it different enough to worry about? Not sure of the answer, but thinking about it I would think the float height is only going to change a % of it's diameter. Might run an experiment with the float in a small container of fuel and then water and see if the difference is apparent and measurable.
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Old 10-01-2020, 10:33 PM   #5
Phil Gillespie
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Default Re: Gas Tank Fuel Gauge reading question

What will affect the postion of the float in the liquid it sits in is the liquids Specific Gravity.
This also changes with temperature.
But in this case maybe using 25 degrees celcius
SG of fuel is .713. as opposed to water at .997.
Float will sit higher slightly in the liquid qith the higher SG.
In this case will not make a lot of difference as will be constant for liquid being used.
ny thughts.
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Old 10-02-2020, 08:13 AM   #6
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Default Re: Gas Tank Fuel Gauge reading question

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Hi Jim,


It will make a minuscule difference. The main height in whatever fluid will be the same. The only difference will be the float itself and how much it will displace the fluid. Water has higher density than gasoline so the float weight won't displace as much water as it does for gasoline.


Glenn
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Old 10-02-2020, 12:15 PM   #7
19Fordy
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Default Re: Gas Tank Fuel Gauge reading question

Thank you all for taking the time to answer my question. I never got good grades in physics so your info. was very helpful. Nice and simple.
Jim
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Old 10-02-2020, 12:40 PM   #8
Lawson Cox
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Default Re: Gas Tank Fuel Gauge reading question

What a useless discussion. What difference does it make? The car ain't going nowhere on water, regardless of what the gauge reads. ROFLMAO
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Old 10-02-2020, 12:49 PM   #9
19Fordy
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Default Re: Gas Tank Fuel Gauge reading question

Lawson, Not useless to me. I wanted to use water because it would be a lot easier to fill and drain the tank in the process of recalibrating my fuel tank sending unit by turning "ON" the ignition switch, not driving the car. I just wanted to know if there would be a big difference in readings obtained using water.
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Old 10-02-2020, 02:26 PM   #10
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Default Re: Gas Tank Fuel Gauge reading question

The accuracy of the 80 year old technology in that gauge is significantly less than the measurable difference in the float levels with those specific liquids.
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Old 10-02-2020, 04:16 PM   #11
19Fordy
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Default Re: Gas Tank Fuel Gauge reading question

I now agree. I ended up draining the tank and then refilling it 1 gallon at a time checking the gas gauge reading after each gallon of gas was added. Thanks again to all who responded.
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