Hey all,

My old opel had pretty much an identical vdo sender (just much shorter) and the vdo gauge was non linear. It read just about dead on (same amount of fuel in the first half as the second. Still trying to figure out the gauge in the 914. Took a long drive today to check on mpg highway (28mpg mixed driving, 2/3 70mph 1/3 60 through the hills). when I filled up this morning it was at 3/4 and took 3 gallons. I don't think I could have slipped another gallon in there although I don't like overfilling so I didn't try too hard.

What have you found, particularly those of you with multiple 914s. Is it pretty much the same car to car? Are the gauges linear across the gauge? How many gallons left when it hits 2/4 and E?


Here is a pic of the gauge from an opel gt.

Click to view attachment

I recently ran out of gas on the NYS Thruway not too long ago, and she made a trip home on the flatbed. I was waiting for the low fuel light to come on. Not a smart move with a reasonably new car. I had a few spares and have found the gas gauge is close to being linear sometimes. I also do not like to over fill so that is a variable. Quite archaic the technology with our fuel senders.

Fuel level sensing is rarely linear.

Complicated subject that is dependent on fuel tank geometry, tank baffling, type of float used, whether analog (like a 914) or modern which will have an analog sensing but will be processed and displayed digitally. Electronic calibration and ability to easily change gauge damping are big advantage on modern cars. Some OEM use lots of gauge damping, others very little. Some OEMs calibrate to leave the gauge hang at full longer to improve customer perception of fuel economy right after a fill up. Seriously. . . Hard to believe but this trick is used more often than you think! Shows up in data analysis of JD Power data (customer perception of economy) vs. EPA fuel economy (measured via standardized test & reporting).

914 system in general is pretty linear due to the design of the float which rises and falls in a straight line (i.e. linear) rather than a float on a long swing arm that travels in an arc. The non linearity comes into play at lower fuel levels where the 914 tank is tapered on the passenger side. I think the resistance pads compensate this to a degree but I’ve never plotted the calibration for myself so I can’t comment with certainty. When my 914 was a daily driver, I felt the gauge was pretty linear - never had issues running it to almost empty after low light came on. Age, wear on the resistance pads, and corrosion would certainly be more of a concern now than it was then.

With respect to it being archaic - even new cars use the same resistance based analog sensing. Ultrasonic sensing is feasible but is more costly and requires a lot of digital processing to give a stable & usable reading. Saw ultrasonic sensing used in a DoD application - I was not impressed.

For a 914, the linearity depends a lot on the condition of the resistance wires and slide contacts inside the sender tube, plus the condition of the wiring back to the gauge itself.
Make sure the low fuel indicator works, and when it comes on, find a gas station soon, no matter what the guage says.

Like @Superhawk996 said, I suspect a properly functioning sender is probably close to linear but the fuel tank is not. Most of the volume is at the top. My gauge seems to work quite well and I would say a little less than 2/3 of the fuel is consumed before the needle hits the half tank mark. I'm really careful not to overfill the tanks on my newer cars but I top up the tank on the 914 because there really isn't much chance for fuel to make its way into the charcoal canister to foul it.

As for the reserve light, mine works great and I use it all the time. I can count on 25-30 miles of range after the light comes on. Perfect for me because I live 15 miles from the nearest filling station so if the light comes on when I'm close to the house, I know I'll have enough range to get to the station the next time I drive.

Some people live in fear of running out of gas, but this is me :


https://www.youtube.com/watch?v=TuEdU_lrtZk

https://www.youtube.com/watch?v=TuEdU_lrtZk

+1, With Kramer at the helm, how could you go wrong? thanks for sharing

See mine is the opposite. The first 1/2 of the gas seems to disappear quick during the gauge, but the last 1/2 of the tank will last for over 100 miles. When my gauge is reading 1/4, I really have 1/2 a tank remaining.

Maybe I need a new sender.

Zach

My gauge, and I have a new sending unit, requires me to fill the tank right to the top to read full. Even then it is just touching the full mark. I suppose I could pull the sender and adjust the float down a tiny bit to get it to read a bit more past full when full, but I never run the tank to empty anyway. Usually stop and fill at about 1/4 tank.

1/2 tank of gas should be getting you close to 200 miles range @ 25mpg.

Likewise, the last 1/2 of the tank should get you well over a hundred miles - closer to 200.

At full tank the fuel sender is low resistance and moves toward higher resistance as the tank is drained.

Before you try a new sender - make sure the contacts at both sender and gauge are clean and that the wiring measures close to zero ohms from end to end. High resistance at the plug / spade contact points and/or in the wiring itself will “trick” the gauge into thinking fuel level is lower than it actually is.

@vaccarabite

I don't let the tank get less than 1/2 full before filling up. I do this with my 914 and 930. On such an old vehicle there is no compelling reason to run so low on fuel. I don't want to find out that my low fuel light doesn't work or the last 1/4-to-empty part of my gas gauge isn't linear or isn't working.

This is good policy for more modern vehicles, too. Most dealer mechanics will tell you privately that their in-tank fuel pumps can be prone to failure if consistently run at or close to empty. I think it's from using cheaper parts and counting on the fuel to cool the pump. These days I usually stop and fill up when I hit 1/4 tank.

Another thing... if you somehow got a 911 sender in your 914 (it can be done) the level will be grossly inaccurate. The 911 tank is lower and wider, and the sender is shorter. So if you have a 911 sender, your gauge will be empty even though you have about 1/4 tank of gas left.

I did some fairly careful measurements on my gauge and tank some time ago. Here's what and how I measured:

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and here's what that data looks like when plotted.

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When I did it I was surprised that the curve was so nearly linear, but later, after I had the tank out for other reasons, I realized that the majority of the tank is very close to a rectangular solid shape and so the capacity vs fuel level is of course going to be nearly linear. The oddly-shaped portion of the tank at the bottom probably doesn't hold much more than 2 or 3 gallons, 1.6 gallons of which is the reserve which isn't sensed by the gauge anyway, and you can see from the plot that the only real deviation from linearity occurs at the low capacity (high sender resistance) end.

Nice! Now that's an answer.

Dan

bookmarking that plot

Holy smokes. Actual data! I love it.

And for those who refill at half or quarter tank, you are missing out - life is more exciting below the 'E'

Wow Larry! Thanks!

Nice to know the gauge can't even see that last 1.6. I was quietly wondering how real that low end of the gauge was. My mustang will sit down there with the ignition off but if you try to run it to E you really do run out of gas.

Tell Betty that... she is always telling me that we need gas the minute the needle passes below the half full line.

She also brags about the fact that she has never run out of gas.. but she always makes me drive if we are getting close to empty.

I'm likely to need a new sender, so started looking at this. The 914 PET shows 90174180100 for the sender and it's the same number for 911s, at least earlier ones. Searching for one online, most places also show the part for use on 911s and 914s. Is there a different number for a proper 914 sender? I be confused.

Dan

C'mon Betty, take a walk on the wild side! Just remember that every gallon of gas adds 6 horsepower robbing pounds.

I use my fuel gauge in combo with the odometer. I reset the trip odometer at every refill and have a good idea of the range I get on a tank. I do have an old car with a gauge that sticks between half and 1/4 tank on the way down. That car is no fun because I spend too much time at the gas pump keeping it full.

my sender and gauge are dead on.....I do need to install a 356 sender and appropriate gauge this winter. I will need help. MarkClick to view attachment