I got about 17MPG when I had the 3.73 gears. Now I get 15-16 with the 3.55s(did not expect that drop). My mileage is predominantly highway as I rarely take the built truck out for any reason other than to race.
about a 200 mile drive. all freeway doing about 60 got me a tad over 19 mpgs. but thats not my average. lol. It would be hard to get a good measure doing city driving with our trucks. Unless you could drive easy from every light and every stop, but I doubt any of us could do it. A few wot pulls or some spirited driving and your mileage will be shit. even though overall our built motors will still do pretty decent if drivin easy. theres no reason for them to be much worse than stock. Except under boost of course.(way worse but more fun)
17mpg hwy cruising at 70 / 12.5 city - used to get around 14.5 city, but that was before the 2600 stall torque converter. Seriously thinking about putting the stock converter back in now that it's no longer raced.............. (the 2001 Lightining).
When I was breaking in the new engine on the 02, I got about 6 mpg - no fun on a 10 gal tank.