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Aero drag is a square function of speed. At 60 MPH squared you have 3600 units of drag. At 70 MPH you have 4900 units of drag or 4900/3600 more drag for a 10 MPH increase in speed.
Aero drag is 70 % of your total of losses due to all factors combined at higher speeds.
With 36% higher aero drag at 70 its easy to see why mileage will suffer, although it is offset slightly by higher sustained load on the engine which improves efficiency slightly.
Not many here will own a brand new car but I have driven close to 80k miles in CVT equipped cars. Your mileage is very good considering the vehicle. A lot of people here will tell you you should have bought a manual, but there are many good reasons to choose an auto.
You would probably get 36-37 MPG if you slowed down to 60 MPH but that may not be possible without increasing the danger to yourself and other occupants. I did find neutral coasting (NEVER WITH ENGINE OFF) to see if it makes a significant difference, but I might not do that on a new car with a warranty, unless the manufacturer approved it an clearly stated it would not be an issue in case of a warranty situation.
regards
Mech
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