Heck the old Oldsmobile deltas, Bonnevills and Lesabre's and other full sized sedans from the late 80's and early 90s were rated for about 18 city but got 28 sometimes 30 MPG on the high way, but the front ends were also not as tall, and the cars were lower in profile it seems.
Hell my 4 door 96 Cutlass Supreme gets 32 on the high way, and about 20-25 in town. But I put a cold air intake on it, stock intake would get me 20 city and 29 highway. It has a 3.1 and a 4 speed over drive trans. Everything else is bone stock.
Now my sisters 93 Grand am gets probably 25 max on the highway, bone stock 3.3L v6 and 3 speed tranny. That trans really robs it of fuel mileage. But both the Oldsmobile and Pontiac have lower front ends though and aren't as tall as some newer cars tend to be.
Now I wonder what both cars would do in terms of power, torque and efficiency with 5 and 6 speed transmissions?