Well you could go with a 3:44 final ratio and get away with 4 speed and 70mph at 3K rpm. I had that in my IM 2110cc and it was a good tranny. In any case with a typeI engine you cannot go below 3K rpms at 70mph so 5 speed is only 4 close gears and 5th overdrive but it has to be done so the rpm stays around 3K for highway cruising.
In my new DDriver, it turns below 2K at 70mph. that would kill an aircooled engine.