You have neglected to give us one important detail- - - - -What engine are you running?????????? Stacking up tall tires on top of an OD gear of any kind is going to create a lugging situation on any gasoline engine. Also, overdrive, whether in a Brownie or integrated into the main transmission, is a power and torque hog. A F-350 wrecker owned by the towing company I used to manage had a 5 speed OD transmission and a 460 V8 engine. Running in OD with a vehicle on the hook almost doubled the fuel consumption over using 4th. gear (direct drive) for the same trip. If you're willing to install some sort of Diesel engine that grunts along at sub-2K RPM ranges, your idea might work, but trying to make even a GMC 302 pull that load at those speeds is a pipe dream. Just because it works on paper doesn't mean things are that simple out in the real world where the too-tall rubber meets the road. Once you get below the effective torque range of the engine you'll be rowing it with that Brownie lever, dropping out of overdrive to direct or underdrive every time somebody even whispers "hill".
Jerry
