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.