I'm at 13k and my truck is just feeling "settled" now.

 

Mine is still relatively new, but I just hit 2800 miles and had the best MPG ever for a tank. Averaged just over 14 MPG this last tank. Ive notice smoother shifting and better throttle response as well. I too have cheated as I have the Flowmaster 40 Series SI/SO muffler installed. The truck is really starting to grow up.

 

My 4.6L is approaching 12k miles and I've never noticed an increase in power. It has been getting a modest 14 MPG since day 1.

I have, however, recently noticed that the transmission shift points seem odd when driving 25MPH. There is an occasional "clunk" when pressing on the gas to maintain 25MPH.

 

well my truck broke at 1352 miles.. thats when ford just replaced the entire engine.... now im going through the break in process all over again!

 

My transmission is doing the same thing but It only has 185 miles on it. Bought it brand new a few days ago. Should I be concerned? Or is it just the tranny breaking in?

 

I never even dealt with the break in thing. I never noticed anything funny about it either.
I get great gas milage in the summer, about 21 mpg average with the 4.6. in the winter it drops down the 15, but I attribute that to the fact that I'm a heat *****, and in the really nasty snow I make full use of the lower gears.