I rented a brand new Chevy HHR a month ago that did exactly the same thing. The guy checking me out pointed out it had a half tank and I needed to return the car with the same level. I drove it a total of about 450 miles. The gauge advanced to nearly full in about 220 miles and and I filled it. The gauge went to empty, and then crept back up to a little over half full before I returned the car. Fortunately, the computer’s “miles-to-go” readout worked just as it should have. Of course I turned in the car after the rental shop closed and I never did get to talk to them about it.