I just got back from a 400 mile trip with 70MPH sections of highway, some two lane and some stop & go. I did the trip in one tank and averaged 17.3 MPG. My truck is a stock 06 XL long bed 4X4 w/4.6 and a fiberglass cap.

 

My 4.6 never got anything stellar for mileage...around town, 13 was average...rarely, if never cracked 17 highway at 65-70 MPH.

The 5.4 in my new truck gets a good bit more...2-3 average better MPG around town and on the highway if i drive it conservatively...and more power on tap when i need it!

 

ya the 4.6 is the best small v8 from the main manufacturers but its def worth it to get the bigger and better motor. my 5.4 avgs arnd 17 on the street and like 13 in the city, but my truck is 4wd and i usually drive like a bat outa hell

 

The 4.6 is probably a fine engine but I would sooner have a 5.0