I put thousands of miles on two different 1970 A-bodies when I moved here to PA & I was surprised at the mileage difference.

1970 Dart, 225 auto with 2.76 gears 14.5 MPG

1970 Valiant, 318 3-speed manual & 3.23 gears - 21 MPG

Both engines bone stock with under 38K miles.