I have a `99 5.4 L Expedition and I am having trouble with figuring out what is wrong with my car. For example, the other day my miles to empty reading said 19 miles left, I then reset my trip log on the dashboard to see how far I actually went before it hit zero. The problem is I only went 6.8 miles before it was on zero. I am only 17 so I don`t know a whole lot about the finer parts of my expedition, so I took it to a mechanic who told me my car was only running on 7 cylinders so I had all the spark plugs replaced ( first time since my parents purchased the car in `99) and the #2 coil. So when I got my car back I thought that would fix my problem but it didn`t help it at all. Yesterday I put more gas in and I was at 273 miles when I left the gas station, I reset my trip gauge and when I had gone 12 miles my reading said 255 miles. I do not understand what is going on! Could it be a leak in my gas tank? A clog in the fuel pump? I honestly have no clue, so please help me if you can! Thanks, and sorry if this is confusing to any of you. P.S. I did not take the car to the mechanic purely for the fact this was occurring, there were several other things wrong with the car as I have only been able to drive it sparingly since I replaced the alternator a month ago.