Civic EG

I got at b18b in my 93 hatch and i am just gettin very crappy gas mileage. Ive notice a couple things. My AF guage stays on rich and goes to stock when I let off of the gas. Also when I think its real close to Empty i fill up and usually comes to around 8 gallons and i am pretty sure that my car has a 10 gallon take. And whoever put the motor in b4 i bought the car cut the harness for o2 sensor and put a single wire o2 sensor in. So what do u all think its could be? Your input is appreciated.

First, your AF ratio gauge, is it narrow band or wide band? narrow band only reads over a half volt range so they are not too accurate. But then I saw the part about the 1 wire o2, I think you should switch to a 4 wire, I'd guess that is a good reason you're running rich. I am pretty sure I saved a how-to on this, let me look around for it.