Had a complete engine rebuild of my 2002 3.0l v6 4x2 auto. At 500 miles into new engine, I topped off fuel tank, drove 100 miles, then it took 8.84 gal to top off tank again. This divides to 11.3 miles per gallon. Does mpg improve with break in? I was expecting18-20 maybe 15 is more reasonable for a king cab with camper shell and bed full of detailing supplies / equipment. I tow a trailer but haven't hooked up yet since rebuild.

 

According to Fuel Economy Index , you should be getting 17 city and 22 highway. This is under optimal, empty weight conditions I imagine. I'm doing good to get 15mpg in the city with optimal conditions in my 95. Using your method to check mpg is ok, but I'd do it several times for a better average.(using same gas and octane) Also take in to consideration the pump you are using. Not all pumps are the same, (may click off at different times) so sticking with the same one helps with accuracy. If you have access to a scan tool, it can give you a lot of info about fuel consumption.
Fuel Economy Index