Don’t you mean that often forgotten nut behind the steering wheel? 
Fueleconomy.gov reports a user that claims to get 20 MPG with 90% highway driving. Then again, the lowest reported mileage was 13 MPG, also at 90% highway miles. The high was in Texas and the low in Idaho. I suppose it could be a combination of terrain and driving habits, but it appears that 20 MPG is possible.