Since a Diesel of that era had only a 300 rpm operating range, anything more than a 15% split in your higher gears will make it unroadable. That is you could never upshift on level ground. you would have to wait for a downgrade to upshift. For example, if you run it up to 2100 and try to go to overdrive theoretically you would come in at 1800 on an upshift which is the absolute lowest rpm you can demand power from engines of that era. This is why governors were usually set at 2300 because you will loose at least 100 rpm in the shift due to wind drag and rolling resistance... even more on a slight incline. The 5 speeds with a bigger gap between 4th and 5th were designed for what we called a working 5th. The 2 speed axle was closer ratio. The upshift sequence was 4th low axle, 5th low axle, then back to 4th high axle and then 5th high axle. There is much more flexibility in gasoline engines as far as gears go than Diesels. If you look at the torque curves of today's Diesel engines and those of 50 years ago, there is no comparison.
