What is the deal with fuel gauges?

[MarkC]

My Cherokee has one of those big fuel gauges,with 20 hash-marks on it. Since it has a 20 gallon tank, you'd assume each mark represents 1 gallon.
I can fill up and drive 100-150 miles with virtually no movement of the needle. Then I'll start up one morning and suddenly, it shows 3/4 full. Seems to actually read accurately from about that point on down to about E, at which point it will still have a couple of gallons left. If I fill it up when it reads half full, it will actually take just about 10 gallons.
Why the delay in reading after I fill it, until I run it a few hundred miles? Is it so hard to make an accurate fuel gauge?

 

[MONKEYMAN]

Maybe you have a problem with the sending unit? Mine was replaced on my Sentra when my gauge would only go to 3/4 tank of gas. Pretty accurate/responsive now.

 

[medic]

I think every vehicle I have ever had has shown 1/2 tank with about 2/3 of the miles it takes to get to the empty mark.

For example: my current F-150

3/4 mark at about 100 to 150 miles
1/2 mark at about 200 to 240 miles
low fuel light (1/8 tank) at about 300 - 330 miles
empty mark - 330 to 360 miles. (about 2 gallons left out of the 30 1/2)

I guess you just have to get to know your vehicle and it's personality

 

[Ugly3]

Every fuel guage I have ever seen works like that.

 

[ekrampitzjr]

Ditto here. The half-tank reading with only a third of the miles actually left on the tank contents seems to be the norm, even for most imports.

 

[hk33ka1]

Try running a gas station. When a guy has a 10 gallon tank and his guage says half and you sell him 7 gallons he will say your pumps are rigged and you just ripped him for 2 gallons. Some people think that their guages are a precise scientific instrument.

 

[Palut]

Since its often a floater device sending the fuel level, the shape of the tank can have a big impact on fuel reading. If the tank is full, and the filler tube is full above that, the gauge would show full until enough gas has been burned to empty the filler tube.

 

[michaelc80]

quote:

---

Originally posted by Palut:
Since its often a floater device sending the fuel level, the shape of the tank can have a big impact on fuel reading. If the tank is full, and the filler tube is full above that, the gauge would show full until enough gas has been burned to empty the filler tube.

---

I think you hit the nail on the head about the shape of the tank. I doubt most sending units are calibrated to take that into account, I know the LTL trucking company that I used to work for had a lot of trouble with drivers running low/out of fuel, because the round tank gave a false reading as to how much was really there when it was 1/4 full. Drivers could go a lot further on the "second" or "third" quarter, than they could on the first or final quarter.

 

[tweeker43]

many fuel pumps are contained within a module that has the level sender mounted to the side. the module ensures that there a supply of fuel at the pump intake during acceleration, stopping, hills, etc...

level senders are generally calibrated to show less fuel towards the end of the tank to "encourage" people from running the tank dry. going dry can cause a turbine pump to spin weld itself together, or make it difficult for a gerotor or roller vane pump to pick up fluid.

hot fuel will swell, causing readings to be slightly exagerated, while start-up readings may be low due to the reduced volume of the cold fuel.

contaminants in the gas can coat the parts of the level sender, causing false (or no) readings.

shape of the tank definitely comes into play.

 

[SteveD2]

I've just filled up my Cherokee on every 1/4 mark, so I know exactly how many gallons each represents. I have a 20 gallon tank and at 3/4 I've used about 6 gallons, 1/2 I've used 8 gallons, and 1/4 about 14 gallons.

 

[Patman]

quote:

---

Originally posted by Ugly3:
Every fuel guage I have ever seen works like that.

---

Every car I've owned (out of 16) has been that way too, with the exception of my current car (98 Corvette) and my 98 Firebird Formula. Both of them have awesome fuel gauges, when it says 3/4 tank, it's really 3/4 full. When it says 1/2 it's 1/2 and when it says 1/4 it's 1/4.