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It's a learning process, if you hit reset/ reconnect battery it's dumb. So if you're on a fresh reset and drive like it's stolen hance the 9mpg or if you have a long history of pure idle or driving hard 9mpg.
So reset drive on nice flowing 65/70 mph traffic for 100 miles 27 mpg.
Unlike newer cars that reset when tank is filled these don't so you can have a lifetime average or last 30 seconds
I had a similar issue so I pulled the fuel pump from the tank, moved the pump unit and saw very little change. Next I opened up the sending unit and cleaned it with WD-40 and Q Tips. Reassembled and it has been working perfectly ever since