On my way to work this morning, I had a strange issue. My fuel notification came on last night, right before I got home, and I figured I could get gas on my way this morning. Well I had driven maybe another 10 miles, and I hear the ding, out of curiosity I check the DTE and it says "0 Miles to E", but my gauge was showing a little above an 1/8 of a tank. I've had it lower on the gauge many times (my wife likes to only put in a little gas, which means I always get it with 1/8-1/4 tank...) and the DTE seemed fairly accurate at that time. Usually shows 50 miles when it first comes on, and I rarely have had it touch the red line, but even when it's close I still show 10 miles or so. My fuel economy has been dropping (I figure it's from warming it up in the mornings this winter), but I have a CEL which is a vacuum leak I've been chasing. I have fixed two different vacuum leaks in the last six months, but then something else ends up leaking. Any way the two are related? I don't know how it calculates the DTE and MPG exactly, but figure the vacuum system probably plays some sort of role. Any help would be great. I'm getting to the point where I'm tired of this thing and am starting to resent it, as every little thing seems to go wrong right after I fix something.

EDIT: To clarify, its my 2003 Navigator 4x4, with about 156k miles.

 

You have to remember that the DTE is based on the Running Average Fuel Economy (RAFE). This means that the DTE is only based on the last 500 miles of driving. So for instance let's say you take a 500 mile trip down the highway and average 17 MPG and then begin driving all city at 10 MPG. The DTE will still reflect the 17 MPG highway mileage until it is realigned with current driving conditions. In this example it would show a farther DTE. If you were to drive all city and then take a highway drive you'd see a shorter DTE. So it may not always be accurate.

​​​​​​I just changed the fuel pump and sender in my 2006 Expedition and I was able to drive past 0 miles to empty. Now, with the new pump and sender I get to 10 and she's about out of fuel. So if your fuel sender is getting worn out this may contribute to erratic operation.

The safest bet is obviously to rely solely on the fuel gauge.