/6 with 4 Speed OD, rear gear recommendations?

My Feather Duster is nearly the same setup (mine has the aluminum 833OD, I didn't realize they could come with a three speed). After I had the flywheel turned and replaced the clutch at 288000 a year ago last summer, I checked my mileage. 31.4 running 14 inch wheels with 205/75 tires. Not the 36mpg the factory rated them, but not bad considering the motor has never been touched. Around town mileage seemed to hang around 20 or so. A short time later I put on my 15 X 7 cop car wheels and 235/60/15 TA radials (same diameter as the previous 14s, but considerably wider). I haven't checked highway mileage since the change, but the wider/heavier wheels/tires seem to have cost me a couple of miles per gallon around town.

The 100 pound heavier 8 3/4 would probably cost mileage the same way. While 3.23s or 3.55s might not cost much, if anything, in town mileage, the highway mpg will probably drop considerably. 2.45s or 2.76s most likely wouldn't do anything for you but lug the motor down and burn out clutches.

Installing a T-5 (weighs about the same the all aluminum 833) behind the Feather/Lite slant with a similar OD ratio and a 3.50 or 3.35 first gear would give about the same acceleration of the other gear sets and not give up any mileage up top. The extra gear of the five speed might even improve the around town mileage a wee bit.

An 8 1/4 rear end would be plenty strong for the slant, is also available with a 2.9 ratio, and is nearly as light as the weak suck 7 1/4.