Maybe check your catalyic converter to make sure it is not plugged up? This can cause severe MPG loss. You may need to take it somewhere where they can test it. Is your D a 4x4? Maybe it is time to change the tranny fluids.

I am not sure if it is just me, but when I changed my tranny fluids, I got an extra 1-2 miles to the gallon for the first couple of tanks, meaning maybe the first 400-500 miles. Then it drops back to normal mpg's I would usually get. At first, I thought this was coincidence but after changing the tranny fluid on my 2nd time go around and did the same thing, I wonder why it gets better gas mileage only tempoary?

 

OH LAWD!!!! LMAO.....

 

Duke, where are you located? If you are in a cold climate you are probably on winter gas right now. If you do a search on winter gas you will see that it causes poor gas mileage.

 

I was getting some bad gas mileage at one point with my 98 D. I put a K/N style intake which helps a little but the biggest change I saw was after I had a shop clean out all the built up carbon inside the engine and new spark plugs.