I have a 94 f-150 351W. if i fill up both tanks at the same gas station my front tank gets abot 12 mpg and the rear gets about 7 mpg. also when i switch from the front tank to the rear tank i notice a huge loss of power. i haven't checked the fuel pressures yet but is there anything else it my be. all input is greatly appreciated.

 

Put a pressure gauge on the Schrader valve on the fuel rail and compare the pressures from the two tanks.

Are you getting any crossflow between tanks?

 

Sounds like the shuttle valve in the front FDM is bad, and the rear pump is weak. Read these captions: