The Stovebolt Site Forums The Parking Lot Big Bolts 2 speed rear diff. question. (video)

I think that you said that your single speed was a 7.20:1 or something? I think most 2 speeds were 6.40/8.72:1 from what I've read. Figuring that, at 65MPH the two speed truck with 9.00-20 @ 520RPM would be turning 3,600RPM. 7.20:1 gives 58MPH @ the same 3,600RPM. It's a 12.5% reduction. An option I'd look into for an additional overdrive would be an old auxiliary or "brownie" trans. With a .85 OD brownie and the 6.40 gears you'd only be turning 2,800RPM @ 60MPH and a little over 3,000RPM@65MPH, plus even more gears if you use the truck as a truck.