I have a 2000 F-150 4x4 5.4L auto, and installed a Flowmaster 50 Series SUV single in, dual out about 2 months ago. Since the installation, I've noticed my gas milage go down, as well as low end power. I've unconnected the battery to reset the computer, but didn't notice any changes.
I also noticed that I don't have to hit my overdrive button, until I've reached 65 MPH, where before I had to turn it on at around 50 MPH.

Am I doing something wrong? Is there something out there to correct this?

Please help!

 

You just pushed the power higher up in the rpm band. Since these trucks are so torquey in the low end already, losing a little low end to benefit the upper end is a good trade off if you ask me. I put a SI/DO catback on a Silverado truck and lost so much low end it was like starting out in 2nd gear but that is because they are so weak in low end torque to begin with.