Just 2 months ago I had to replace all 8 injectors plus my fuel pump. After I noticed my gas mileage was getting ALOT better (19-20 mpg on hwy). Well it was going fine all up until a few days ago when all of the sudden I was running a higher RPM than normal and my idle was a low RPM. Well then my gas mileage too a huge hit all the way from 18 mpg to 8 on a good day. I really don't understand what's going on here and it's quite frustrating.

 

if your tires are too wide or bigger than factory you can swap them out or maybe something with less tred. little extra mpg can come from a nice tuner with some good custom tunes and maybe an extra 1 or 2 from cold air upgrade and different exhaust. if owner before you put bigger injectors check those. believe it or not i had a old rubber gasket on the fuel pump underneath middle of truck not one in gastank or that sits above and for some reason it gave me 3 mpg better with getting a new one. was told those gaskets go often on diesels cause there under so much pressure especially if u tow or plow ect...

 

What I don't understand is how it can drop quite significantly at random. I JUST replaced all 8 injectors! And we are getting the transmission looked at tomorrow also. But another issue my engine sometimes dies on me when I put it in D. It will surge really bad as the RPMs drop and it will die. It was doing that right before my injectors went out, I doubt it's the injectors because they're 2 months/2k miles old. My FICM is normal voltage, my fuel pump is brand new also. I am quite confused by all of this. Can it be transmission related?

 

i have a 2003 excursion the beast of them all.. i get 19-20 mpg . most that get horrible fuel mileage usually have their hubs locked into manual instead of auto. i get onto mine quite often. if your in auto lock then you may have some issues