T5 Five speed slant six 74 Dart Sport.

....and I bet that thing would have gotten much better mileage with a 3.55 or even a 3.91 and no matter what you do, you cannot make some folks understand that. Are you listening, @greymouser7 ? Some folks think the lower numerically you go with gear ratio, the better mileage you get and that's just not true at all. If you get an engine so far out of its optimal operating range, you ruin any chance of good economy, not to mention decent power. With that 2.76 gear, I bet you were downshifting with the slightest head wind. That had to SUCK.
my duster with 25" tall tires, stock 1bbl slant6, OD and 3.23 got it's best mileage at 60mph (about 30mpg) (about 1900 RPM). 65 it drops down to 27-28, 70MPH more like 25-26.