porsche mystery: assymetrical fuel tank level gauge

[gliding_serpent]

Just wondering why my 84's fuel level gauge is asymmetrical (i.e. the distance between the top 1/4 tank is much larger, and it decreases until the much smaller bottom 1/4th)?

My guess is that the fuel level meter is linear, but the shape of the tank is not (larger capacity at bottom of the tank than top), thus the non-linear fuel level.

[DRACO A5OG]

Hmm? I always wondered too, it must be a German thing, "being literal" like our lovely Oil Level Gauge

[Neilnaz]

I imagine it is an artefact of the profile of the tank - less volume at the bottom 1/3 compared to the mid and top 1/3s. There may also be a factor with the mechanics of the float lever as it drops with non-linearity as it goes through an arc of motion..... Actually I don't know the mechanics of the fuel sender...

Cheers, Neil

[twistoffat]

I imagine its to do with the float too. With the tank full and despite using fuel you wont get any reading until the fuel level drops below the level of the float and it begins to sink. When it does drop then its suddenly noticeable. I figure that you only really get accurate readings for the last 2/3´s of a tank.

[john walker's workshop]

I always thought so. Just when you thought that last 1/8th of a tank would get you home, poof, it's reading on empty.

[McLaren-TAG]

I don't know, I've never filled mine all the way.

[Joe Bob]

I ignore mine as well as the oil level gauge. I go by mileage and the dipstick....remember VDO was the low bidder.

[T77911S]

Originally Posted by john walker's workshop

I always thought so. Just when you thought that last 1/8th of a tank would get you home, poof, it's reading on empty.

yea, you think you are getting really good gas mileage because you are only at half.

its like from full to 1/2 is really full to 1/4.

[gptom]

It's more important to know the last quarter tank accurately then the top 3/4

[Cattler]

Originally Posted by gptom

It's more important to know the last quarter tank accurately then the top 3/4

I agree but have a different perspective--I thought it was to reinforce that you should fill up before you run down below a quarter of a tank. A purposeful design feature then, not a flaw.

[Jcslocum]

All gas tanks have a big half and a small half. You just need to understand the last 1/4 and how far it will take you!!

[gliding_serpent]

Just wanted to confirm that the gauge is assymetrical, because the tank is assymetrical. I.e. The top part of the tank is smaller, so there is a larger gap between 3/4 and full. There is lots of volume in the bottom of the tank, so the space between empty and 1/4 is narrow in the gauge. So 1/2 truly means you have 1/2 a tank left, And 3/4 and 1/4 really mean what the gauge said. I feel almost silly having asked.

[TheSt|G]

Originally Posted by Joe Bob

I ignore mine as well as the oil level gauge. I go by mileage and the dipstick....remember VDO was the low bidder.

Shrug, my oil level gauge is laser accurate to the dipstick. YMMV

[gliding_serpent]

Originally Posted by TheSt|G

Shrug, my oil level gauge is laser accurate to the dipstick. YMMV

Mine too as I have discovered. Too little oil and the needle hops all over the place, but since I started doing oil changes myself, I measure close and it is spot on.

[T77911S]

i love the oil gauge. i did a write up on pulling the sender and cleaning it up.
when my needle gets down to the red i drop in another qt., thing is i can check it at stop lights and what i notice is sometimes the level is higher than others. so if i checked it with the dip stick when the tank was not as full as other times, i could put in too much oil.

those that put it down dotn understand how to use it

[87m491]

Not buying that argument, lots of tanks are near squares or rectangles. I have a few of them. Almost perfectly symmetrical horizontally or vertically....

Originally Posted by Jcslocum

All gas tanks have a big half and a small half. You just need to understand the last 1/4 and how far it will take you!!

[87m491]

Not so sure this is the case. I've had a few 80's German cars where the tanks are pretty symmetrical and still the gauges seem to exhibit the slow first 1/2 drop and the rapid 2nd 1/2 drop.
Using your logic, if the top 1/2 of the tank is smaller, volume wise, wouldn't the gauge run through that area faster than the lower 1/2? It would be pretty straightforward for VDO to have calibrated the sender to account for the shape of the tank and have the gauge reflect to true volume left as a true percent of the total usable.

Originally Posted by gliding_serpent

Just wanted to confirm that the gauge is assymetrical, because the tank is assymetrical. I.e. The top part of the tank is smaller, so there is a larger gap between 3/4 and full. There is lots of volume in the bottom of the tank, so the space between empty and 1/4 is narrow in the gauge. So 1/2 truly means you have 1/2 a tank left, And 3/4 and 1/4 really mean what the gauge said. I feel almost silly having asked.

[gliding_serpent]

Originally Posted by 87m491

Not so sure this is the case. I've had a few 80's German cars where the tanks are pretty symmetrical and still the gauges seem to exhibit the slow first 1/2 drop and the rapid 2nd 1/2 drop.
Using your logic, if the top 1/2 of the tank is smaller, volume wise, wouldn't the gauge run through that area faster than the lower 1/2? It would be pretty straightforward for VDO to have calibrated the sender to account for the shape of the tank and have the gauge reflect to true volume left as a true percent of the total usable.

It should run through everything at the same rate in theory (ignoring using more fuel for a heaver car with more fuel). But the needle will "travel faster" at the top of the gauge than the bottom, but there should roughly be the same amount of gas between each of the lines on the gauge.

When I get in the final quarter of a tank, the needle takes forever to move... Compared to the 1st 1/4, but in the end, despite gauge spacing, and milage differences due to weight, they should have about the same amount of gas.

But this is hardly scientific, but the shape of the gas tank supports my non scientific observations. I could be wrong however, but i just report what I think I see.