I have a similiar story. I had a 2000 Ranger with the 4.0 V6, I got around 18.5 mpg average over the 3 years I had it. My 2004 F150 with 4.6 V8, I get around 17.9 mpg since May.

 

I have put 600 miles on my truck this week, filled it twice. A lot of highway, maybe 75 percent, a mix including 30 miles or so creeping the freeways, and obeying the speed limit, almost never over 68 mph, much at 55-63. Only a little ac use, only a few WOT moments.
Both fillups returned 19.6 Mpg. Not too shabby.
My Superchips set on 87, with their default shift settings, seems to drop my mileage around town, but may help on the open road. The fun increase from the Superchips is worth the drop.
Chris

 

I've got the 04 XLT extended cab, 4.6L v8. It's taken a while, but with custom exhaust (flowmaster series 40) and having used 89 octane since I bought the truck, now at 9k miles, on the highway I got 17.59 mpg.

Unfortunately, now that I'm back off of vacation and driving around town, I'm back to the usual 12 mpg. I expected to get 15 in the city and 19 on the hwy ... oh well.