The problem is that comfort is at odds with economy (there's no solutions, only trade-offs). The most comfortable vehicle would be a minivan since it's tall and you sit upright and it's designed to spend long hours in the saddle. The most fuel efficient ones get 30 MPG, which is impressive for the vehicle size.
A car that sits lower to the ground and has a more efficient shape will have your feet out front, which is less comfortable.
Audi A7?
