I remember reading a post on corral.net from quite awhile ago, when I had my Mustang (sniff, sniff.... ), about this same topic. This happens in just about every make or model car I have ever had or have driven. Someone had measured resistence values across the sweep range of the "gauge" inside the tank and compared the ranges with the measured fuel usage to determine what the values were and etc, etc, etc. Basically, he determined that the gauge only reads approximate values. It's nowhere near exact. It's built to have some "fudge" factor in it. That's why some cars will be completey empty just at "E" and some quite a ways passed "E".
Found this post from another car forum via google that suggests something similar via Automobile magazine...
"Based on an article I read years ago in Automobile magazine: The needle or digital display for the gas tank does not move in a linear fashion. They are designed to remain at "full" for a while, suddenly drop to half a tank, and then meander down to empty. This is a feature, not a bug, as it is intended to give the impression of greater fuel economy for the vehicle.
In the article, a Cadillac with a digital gas readout in gallons was described. When the car initially shipped, the gauge went from "Full" to 17 gallons as the first gallon of gas was consumed. The company was deluged with complaints about the poor fuel economy of the car. Their fix was to change the gauge from moving from "Full" to 17 to "Full" to 16, thus displaying "Full" for twice as long. No other change was made to the vehicle. The complaints ended.
Caveat: I did read this a long time ago and am too lazy to dig up a citation. I believe this to be correct but, you know, if it's important verify it elsewhere.
posted by stet at 2:40 PM on July 14, 2004 "
It seems to be better now. I got 40 miles before the thing even moved off the Full marker...about the 7/8 mark right now
I'll keep a watch on it, and make sure I get it Full from now on. I cannot stand that stunt where gauges stay full forever, then jump to the next increment
it is weird though how Ford seems to build that "Reserve" into the their tanks/gauges. My Durango is the exact opposite. When it says "Empty"...they ain't kidding. You've got about 1 gallon left and some fumes
07 EL has the same "issue" i've learned to slow the nozzle down after 20 gallons or so then let it sit for a min. If you rock the truck that helps too, get the air bubbles or whatever you want to call it down quicker. I'd say your right that is worth about 2 to 4 gallons depending on how fast the pump goes and foaming properties of the said fuel.
I just filled up last night and the guage read 1/4 so I filled it up and ended putting in 17.527 gallons and the guage read just over full. I have an EL and this has been my issue since I bought it. I have 25671 miles on her. She is an 2008. I cannot figure out what the problem is unless Ford up a huge reserve from a 1/4 of a tonk on down to empty. Craziness.I bet I won't ever get it solved.
LONG WHEEL BASE (LWB) - FUEL GAUGE READS
BETWEEN 3/4 AND 7/8 WHEN FULL
Some 2007-2008 Expedition and Navigator LWB vehicles may exhibit a refueling issue where the customer states they refuel the fuel tank and the gauge reads between 3/4 and 7/8 full.
Follow the Service Procedure steps to correct the condition.
1. Question the customer - yes/no.
a. Refueling event more than a 5 gallon fill?
b. Customer refilled with the engine off?
c. Customer refilled to 1 to 3 auto-shutoffs?
NOTE SOME CREDIT CARDS HAVE PRESET LIMITS ON THE DOLLAR AMOUNT AVAILABLE FUEL TRANSACTION. THE PUMP WILL SHUT OFF AT THIS LIMIT. THIS LIMIT MAY BE IN THE RANGE OF $50 TO $75.
If the answer is No to any question, this procedure does not apply. This is the normal system function. The system may not recognize less than a 5 gallon fill or filling up with the engine on and it will be slow to update the gage to the correct fill level. Also, the first auto-shutoff is designed to read full.
2. Follow Workshop Manual (WSM), Section 413-0 Pinpoint Test A: Instrument Cluster, Message Center, and Warning Chimes to determine if the cluster is capable of reading the 4 fuel levels, using IDS active commands.
a. If not okay, refer to WSM recommendations.
b. If okay, go to Step 3.
3. Follow Pinpoint Test A Section 413-01:
Instrument Cluster, Message Center, and Warning Chimes to determine if the cluster can read 4 fuel levels using the gage tester.
a. If not, refer to WSM recommendations.
b. If okay, go to Step 4.
4. If the vehicle was built prior to 7/1/2008, remove and inspect the fuel pump. Verify sender float rod clearance to convolute fuel supply hose by sweeping the float to the full position. Is the clearance more than 3/16" (5 mm) between the fuel soaked hose and the rod?
a. If clearance is equal to or greater than specified, go to Step 6.
b. If clearance is less than specified, go to Step 5.
5. Follow Pinpoint Test A Section 413-01:
Instrument Cluster, Message Center, and Warning Chimes to determine if the sender card meets the 10 to 180 ohms specification?
a. If within specification, follow WSM recommendation.
b. If not within specification, go to Step 6.
6. Replace the fuel delivery module (FDM) with certified stock. Check for a blue pen marking near the gray fuel supply port on the sender flange.
a. If blue pen mark is present, use the FDM as the replacement.
b. If blue pen mark is not present on the FDM flange, please verify the sender float rod clearance to convolute fuel supply hose by sweeping the float to the full position. If the clearance between the float rod and the convolute fuel supply hose is less than 13/16" (20 mm) on a dry hose, do not use. Order a new FDM and repeat inspection for blue markings and/or clearance before replacing.
WARRANTY STATUS: Eligible Under Provisions Of New Vehicle Limited Warranty Coverage
IMPORTANT : Warranty coverage limits/policies are not altered by a TSB. Warranty coverage limits are determined by the identified causal part.
OPERATION DESCRIPTION TIME
MT081901 Use SLIS Operations If Actual
Available; Claim Additional Time
Diagnosis Or Labor
Performed As Actual Time.
"stuffing" an extra 1-2 gallons in there helps...but the gauge almost invariably reads less than "Full" after about 10-15 miles.
I'm of 1/2 a mind to just let it be though. This TSB describes my issue exactly. But the Ford places are here a ****ing retards and I am not about to get into another repair ordeal like I did with my paint.
I think at the end of the day, it just isn't worth screwing around with it. It is relatively accurate once it reaches 3/4 tank reading...so it ain't like I'm going to get stranded by running out of gas