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If the engine is newly rebuilt it can take a few thousand miles to break in (seat rings,valves, etc.).

My truck's new (rebuilt) 383 stroker got 10 mpg on the first tank, 11 onthe second, then 12, then 13, then 14, etc.

On the advice of Dick Guldstrand (the Corvette guy who works on my old Sting Ray's chassis), once mileage stabilizes I'm taking the truck to his guru, Scott Brown of Claremont, California. Scott has a dyno and megabucks worth of GM tuning software. He can dial-in and optimize power or mileage with your vehicle's OBD computers. He works with NASCAR teams and has made blown 427 small blocks behave on the street when no one else could tune them.