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#4591338 - 12/01/17 06:16 PM Fuel Level Gauge Accuracy
mbacfp Offline


Registered: 09/02/15
Posts: 778
Loc: California
Have a F550 with the 28 gallon front tank and the 40 gallon rear tank. The rear tank was showing 1/4 left and when I refueled it only took 22.5 gallons or so to fill up. Is that normal to be off by so much? Thanks and hope everyone is well.


Edited by mbacfp (12/01/17 06:16 PM)
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#4591349 - 12/01/17 06:27 PM Re: Fuel Level Gauge Accuracy [Re: mbacfp]
Traction Offline


Registered: 06/04/13
Posts: 960
Loc: iowa
They have always messed with fuel level gauges. It was a big problem at one time when the level gauge was probably accurate, and people got upset when the gauge started dropping from the full mark not long after filling up. So, they made them stay on full for a long time, then it would drop like a rock after 3/4. I still think it's that way now.


Edited by Traction (12/01/17 06:28 PM)
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#4591350 - 12/01/17 06:29 PM Re: Fuel Level Gauge Accuracy [Re: mbacfp]
Miller88 Offline


Registered: 04/09/08
Posts: 12628
Loc: Onondaga County
I don't think I've ever had a Ford with an accurate fuel gauge ...
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#4591360 - 12/01/17 06:34 PM Re: Fuel Level Gauge Accuracy [Re: mbacfp]
mbacfp Offline


Registered: 09/02/15
Posts: 778
Loc: California
Thanks guys. Always thought it was off when refueling and started to take more notice. As always, appreciate the input.
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#4591363 - 12/01/17 06:39 PM Re: Fuel Level Gauge Accuracy [Re: mbacfp]
TmanP Offline


Registered: 10/30/15
Posts: 432
Loc: MN
There's what's called a "full reserve" so it stays on F for a while (maybe a couple of gallons). The same is true for "E" - when it indicates empty, you have a little gas left. This is pretty common. The automakers want to have their cake and eat it too: they want to give the illusion of good gas mileage (first half often takes longer to burn because it's sometimes much more than a half) but they want to make it so customers don't run out either. Trucks usually have more reserve because possibility of abysmal mileage (towing, idling, etc).

I noticed you have a Sienna. So do we. Notice how its gas gauge works? It has about six or seven positions that it's always in, it doesn't act even remotely linear. 21 gallon tank, 3 gallons on F, about 4 gallons left when on E, stays close to F for a while, then slowly moves down. From 3/4 to 1/2 is pretty quick, then it gets stuck at the 3/8 mark for seemingly forever. After that, it rockets toward "E", and I've checked that there's at least three gallons when the gauge indicates completely empty. I drove on the light for 25 miles, then put in only 17.7 gallons or so. This means you can drive 100 miles on the light on the highway!
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#4591380 - 12/01/17 06:57 PM Re: Fuel Level Gauge Accuracy [Re: mbacfp]
motor_oil_madman Online   content


Registered: 11/29/09
Posts: 4716
Loc: Houston, Texas
I always thought it was because the fuel drowned the float when it was full, so you had to burn a few more gallons before it went down.
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#4591382 - 12/01/17 06:57 PM Re: Fuel Level Gauge Accuracy [Re: mbacfp]
mbacfp Offline


Registered: 09/02/15
Posts: 778
Loc: California
Yes, I have noticed that as well. My Mazda also stays full for a while then falls all of a sudden. Great info. Thanks.
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#4591479 - 12/01/17 08:49 PM Re: Fuel Level Gauge Accuracy [Re: mbacfp]
justintendo Offline


Registered: 03/04/17
Posts: 99
Loc: northwest pa
sounds like a pretty accurate ford gauge to me. every ford i have had, going back to a 1986 model...always has a few gallons once you hit "E". kind of like a reserve, i always thought they did it so people didnt run out of gas.
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#4591696 - 12/02/17 06:19 AM Re: Fuel Level Gauge Accuracy [Re: mbacfp]
mahansm Offline


Registered: 01/19/10
Posts: 107
Loc: Panama CIty, Florida
With the float on a lever arm with the pivot at the top of the tank and a linear potentiometer
driving the gauge, you get no movement until the float drops from the top of the tank
where it was pinned by the high fuel level. You then get slow movement which increases
in speed as the fuel level drops. Depending on the length of the arm, you'll reach empty
with some fuel left in the tank.

I had a Pinto once that had the gauge above full when the tank was full. Took about 1/4 tank to drop
to F, another half tank to drop to an indicated 1/2, and it was rapid from there down.

In comparison, all the Mercedes I've owned had fuel gauges with the marks spaced unevenly on
the gauge but dead accurate regarding the remaining fuel level. The gauge moved away from full
almost immediately after leaving the station.

My current Honda Odyssey gauge is fairly accurate but the low fuel light comes on with 5 gallons
remaining (out of 21). There's still 3 gallons in the tank when the needle gets to "E".

Last time I fueled my motorhome the gauge was midway between 1/4 and 1/2 and it took 160 gallons of
#2 Diesel (260 gallon tank).
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#4591733 - 12/02/17 07:12 AM Re: Fuel Level Gauge Accuracy [Re: TmanP]
Delta Offline


Registered: 04/20/14
Posts: 1060
Loc: South Central PA
Originally Posted By: TmanP
There's what's called a "full reserve" so it stays on F for a while (maybe a couple of gallons). The same is true for "E" - when it indicates empty, you have a little gas left. This is pretty common. The automakers want to have their cake and eat it too: they want to give the illusion of good gas mileage (first half often takes longer to burn because it's sometimes much more than a half) but they want to make it so customers don't run out either. Trucks usually have more reserve because possibility of abysmal mileage (towing, idling, etc).


Wish the S10 was this way. If you let it get to E, you're in big trouble. That was a fun day...

We had a 1998 Ford Ranger 3.0 V6 at work a while back. The fuel gauge would drop from F in a hurry. Had a 17 gallon tank, but when the check gauge light (aka look at the fuel you idiot, lol) came on the MOST you could put in was 13 gallons. Not fun driving long distances, and I never wanted to risk it.
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#4591743 - 12/02/17 07:20 AM Re: Fuel Level Gauge Accuracy [Re: mbacfp]
SHOZ Offline


Registered: 06/28/03
Posts: 5603
Loc: Illinois
Both my Hyundais are fairly accurate. When the low fuel light comes on there's about a gallon and a half left. On the Gen coupe when the light comes on it's usually around 50 miles to empty displayed, but when the light comes on the display blanks out. Turbo gets limited to 5 psi when the low fuel light is on too.

Accent gets about a 100 miles to the mark and 475 miles to the full tank. I usually figure 75 miles when the light comes on. The Gen Coupe I figure about 45 miles when the light comes on.

haven't ran out of gas yet.....
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#4592432 - 12/02/17 08:38 PM Re: Fuel Level Gauge Accuracy [Re: SHOZ]
bbhero Offline


Registered: 03/20/15
Posts: 4373
Loc: Virginia
My car lies to me all the time. It always has more in it than what the gauge shows. Hard to know how much to put in that doggone car. It's off by at least 2 gallons on average.


Edited by bbhero (12/02/17 08:39 PM)
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#4592493 - 12/02/17 09:37 PM Re: Fuel Level Gauge Accuracy [Re: mbacfp]
andyd Offline


Registered: 09/25/04
Posts: 6873
Loc: Marshfield , MA
Accurate or not, you disobey it at your peril. My '58 VW bug had no gauge, just a 1.1 gallon "reserve tank". It was just a lower pickup with a shut off. The sinking feeling of the engine dying, followed by the relief of the engine restarting was enough to fully impress upon you of the need for gas. grin2
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