First: .725 is way too high. If your check engine light isn't coming on, then there's something wrong with your computer. Lower the EFIE's setting to .125, then after testing your mileage, start raising it from there.
At test number 2, the voltage of .650 should not be a steady voltage, as you point out. Was your voltage changing? It should've been, but I need to know that it was. If it wasn't, your sensor isn't working.
Your 3rd reading has got me scratching my head, but about the only way I can see this happening is if the sensor and computer wires (white and green wires on the EFIE cable) were hooked up in reverse. If you wire the EFIE in reverse, then the EFIE voltage will be subtracted from the sensor's output rather than added to it. This would go in reverse of what you're trying to do.
Please verify that you have cut the oxygen sensor's signal wire and connected the green wire (in the EFIE cable) to the oxygen sensor, and the white wire is connected to the computer (ECU or PCM).
You make no mention of a check engine light, yet if the CEL is on, and it's saying the oxygen sensor is faulty, then the computer will ignore it's information. At that point it will "guess" at the air/fuel mixture, and it always guesses in the direction of too rich. Result is your mpg goes down.
If you are absolutely sure of the above, then, we had better check the functioning of the EFIE. If the EFIE is removed from the car circuitry, you can verify it's functioning by the following tests:
1) hook up 12 volts to the red and black wire (+ to red wire).
2) verify the voltage comes up between the red/black test ports. Test both pairs for a Dual EFIE Deluxe
3) verify that the same voltages are appearing on the white/green wires and the blue/brown wires, as these should be connected to the test ports.
4) Test for a short to the black wire, from each of the signal wires: white, green, blue, brown.
5 While we're at it, make the same test between the red wire and the signal wires.
If these all pass, then the EFIE is working. The fact that you're getting .725 volts out of one of the EFIE ports tells me that the range is set too high. I would go ahead and adjust the range of both sides of your Dual EFIE to .500 or so. See EFIE Range Adjustment Instructions
, for instructions how to do this.
From the information I see here, it sounds like the EFIE has been hooked up incorrectly (white and green wires reversed, or blue and brown wires reversed). However, verify the installation as above, then if no joy, test the unit as described, and let me know what you find out.