MPG Increasing then decreasing on New Truck

[fharsh]

Why would my mileage go from 16. 4 MPG when the truck was new to an all-time high of 19. 1 when the truck had 2000 miles on the clock and then steadily drop to 17. 1 at 3000 miles. All calculations are by hand and closely correspond to the overhead. No change in driving habits. All fuel from the same high-volume truck stop. No leaks. All driving is unloaded except for a single occasion when I towed about 3000# for about 10 miles which just happens to be when the mileage started to drop. I have an appointment at the dealership next week for my first (free) oil change. Any ideas?

 

[Crunch]

Why would my mileage go from 16. 4 MPG when the truck was new to an all-time high of 19. 1 when the truck had 2000 miles on the clock and then steadily drop to 17. 1 at 3000 miles. All calculations are by hand and closely correspond to the overhead. No change in driving habits.

It's still a new truck. Give it till about 20,000 miles before the rings seat and the engine is broken in. And keep the RPMs consistent and below 2000. Anything above 2000 RPM and the mileage drops and becomes inconsistent. If I could keep my foot off the pedal I could consistently get about 20MPG. Unfortunately, I can't, and it don't, but I have averaged about 17. 5 - 18 MPG over the 46K I've racked up since I bought the truck.