FWIW...I have my doubts wether an OD will have much appreciable value with regard to mileage, especially if you choose to run larger than stock O D tires.
I'm rolling 35's, & my mileage got a little better with the addn of 4:77 gearing.
Indicating to me, that the mileage "sweet spot" is 2k & above.
Maybe for the occasional downhill grade, but for flat land running, I have my doubts.
Well....35's do weight a lot, lift the body up, and create more drag. FWIW, vettes with the A4 get 25mpg. With the ZF6, it moves up to 30mpg. Big difference slowing a SBC down (for hwy mpg).