Slant Six Fuel Mileage?

It all depends on the /6 and the configuration AND how, and how hard, you drive it. My family had 5 of them:
'68 Dart Auto: 20-21 mpg 65 mpg interstate
'72 Demon 3 speed: Maybe 1-2 MPG better than above
'73 Demon 3 speed: Same as above
'76 Dart Lite 3speed + OD and all the Lite version stuff: 29 mpg at 65 mph interstate
'80 Aspen Super 6 Auto with locking TC: 24 mpg interstate

Drive them at 75-80 MPH and the mileage drops like any other car. Drive them in urban or suburban stop and go traffic and the mileage drops. Nothing new here....

As for the fuel gauge the OP could get the gauge a bit closer at E by bending the float arm up.