See your /6 Duster has a 7 1/4 rear end.
Lots of those came with 2.76 ratio. Had a 74 /6 Dart Swinger that came 2.76, automatic.
A little taller rear tire will give you actual more miles per gallon.
Kind of doubt you have a 3.23 ratio, but you can check. 2.76 rear gears will be best for mileage if that is your main concern.
View attachment 1715725625
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Would think you could be closer to 20 mpg if set up right with that /6.
Had a '69 Satellite with stock 318, 4bbl upgrade, 727, and 2.94 rear gears. That one got 20 mpg. Everything was set up right working well together.
9.2:1 compression ratio in '69 helped too, more power out of the same fuel.
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