bad fuel mileage after rebuild

Maybe. I know on my 08 Jeep Wrangler it took a while for break in. Right at 2000 miles, it felt like it added 50 LB FT of torque. .......that said, it started knocking at 18K miles and had to have a new short block.
Friend of a friend got a block back from machining, assembled it...he was green, anyhow it was only bored and not honed...it ate itself in ..I think it was 500 or so miles?
Things happen.