Here is my simple alternative to this. A dingle speed.
It has both a 17-21 fixed cog and freewheel on the flip flop hub. The chainrings are 46x42.
This gives me 2 very usable gears with the same length chain. the 21-42 is 55gi and the 17-46 is 75gi. Not the ratios you are looking for but much simpler setup.
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Cambodia bikes, Bridgestone SRAM 2 speed, 2012 Fuji Stratos...