Sachs used to make a 3x7 rear hub coupled with a single chainring for 21 speeds. I was thinking that if you added a double CR it would be a 42 speed and if you added a triple it would be 63.
That seems, to me, to be about enough gears to tow almost anything and go up the steepest grade without having to work so hard.
There would probably be little improvement in top speed, but the lower ratios would be incredible.
I'm thinking that might be my next project. The problem, of course, is finding a frame that won't crumple, buckle or break under the stress of towing heavy objects. Perhaps and old Schwinn would be satisfactory. Any ideas?