Sounds like your engine is fuel starved, check and replace your fuel filter if you have not recently. Also your fuel pump could be going bad, not building enough fuel pressure and with the false readings you are getting on the guage/mpg comes from the fuel pump flow and fuel level sensor. If you are not getting any trouble lights then your problems is mechanical (nee fuel pump) and may not show up on a OBD check which is recommended. I would check with the guys at Stage3, they are knowledgable on this and could make the proper fix.