Gas mileage drop after new fuel pump installed. on 1998 Dodge Ram 1500

My fuel pump decided to stop working one night. Apparently, the electric pumps work well until they quit (unlike the old mechanical pump that would get weak but rarely completely fail, so you could limp home). Any way, I buy a new Bosch unit, and put it in. From that moment on, the gas mileage has dropped. And curiously, the FASTER I drive, the better mileage I get. As in 85 MPH gets me 17 MPG, but 70 MPH gets me 15, which makes no sense . . . unless the same volume of fuel is being pumped through the system regardless of my driving activity. My in-town mileage is around 11. Is this something anyone has experienced? Any one have any suggestions as to what the problem might be? And yes, I know the ultimate solution is to simply drive 85 MPH all the time, but let's pretend that option isn't practical.

Asked by for the 1998 Dodge Ram 1500