Winter gas and winter roads have a huge impact. I have noticed a considerable drop on the freeway when it is raining hard (Seattle) and the roads are very wet, thus increasing the rolling resistance. In the winter it is also much more likely for the car to start, even after a short park, in warm-up mode, which is not a MPG friendly mode. I see at least 28 MPG on both of mine at 65-70 MPH on semi flat, dry roads. A combined tank, typically 70/30 city/hywy usually results in 22.5 to 23.5 which I am happy with.