Truckrolet,
Your 6400 2-speed is most likely the 15,000# Corporate GM rear with vacuum shift. Chevrolet never made an electric shift to my knowledge (the optional electric shift rears were made by Eaton). So third members are not interchangable. The GM rear will have a ratio of 6.40/8.72 and the optional model 16600 Eaton rear has a ratio of 6.50/9.04.
Need to know what tire you have to know the diameter. I think the 10.00 x 22.5 (same as the 9.00 x 20) is about 38" tall.
My 57 Fire Truck with 11.00 x 22.5 tires and a 6.14/8.36 Eaton runs 65 mph all day long and it gets 7 mpg. It has a 200 hp V8 with a direct drive 5-speed trans and weighs 14,200# without water in the tank.
Mike B
