1994 f150 problems

my friend has a 1994 f150 5.0l 4x4 and this past summer he has had the engine rebuilt, transmisson is stock with 190000 miles on it and he is getting really crappy gas milleage. he has replaced the throttle motor(idle),the o2 sensor and mass air flow sensor replaced. all the other sensors are still old. when he had gotten the engine put back in he got about 14-15 mpg and now he is getting 9-10mpg, he has taken it in multiple times and they couldnt find anything wrong, do any of you know what could be causing this?