I have a 2011 Impala LTZ 3.9 flex/fuel It has 9000 miles on it. I just bought it 25 days ago I was told I would get 27-30 MPG hwy miles

I live in FL very flat roads I drive only hwy, 0% city live by the off ramp and work right by the off ramp. use cruise control the whole way there and back. I get 20.1 MPG hwy exactly. and I have filled up in three different gas stations just to rule out bad gasoline.

I took it in and ask the Chevy dealer to please check things out. Right away he flat out told me its the gas I am using and told me he will not find one thing wrong. I got back to dealer and the car was sitting in the corner .

I asked them what they did and He said I system diagnostic and we got no error codes the the car is fine. My Mom has the same car and engine and she lives in upstate NY and she gets better mileage on the hwy with E85 in it. I use Unleaded here since there is no E85 available in my area. anyone have any Idea what going on with this car