oh Hell I never add OD's for fuel milage i do it for road speed tho you still gain some milage only about 2.5 MPG on average. . .
My B61T with the stock Mack tri plex ran 58mph@2250rpm 4MPG with Mikey's version of the triplex it run's 72mph@2,000rpm 6MPG <<< not bad for 1959. . .thats an extra 14mph & 2mpg with a truck that has 6.38 gear's,12.00-20 tires. . .I can still pull D9 cat's with it. . .Due to old tech brake i only run it about 67mph loaded that heavy. . .
Amazing what you can do by adding a turbo to high compression NON-Turbo diesel and altering the trans behind it. . .
I used the Mack single 5 spd from an army truck 5th gear is an OD put a 3 spd auxy behind it's still a 2 stick 15spd but i got dual OVER DRIVE's. . .
thats what I call teaching an old bull dog New trick's

. . .
Mike