Thor has a Boardwalk new for $139 and a Speed 8 for $500 shipped.
http://www.thorusa.com/dahon/accessories/oneoff.htm Utah Trikes has a 3 speed hub ,wheel and shifter for $50
http://www.utahtrikes.com/PRODINFO-S...Wheel_Kit.html
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Speed Uno
Dawes Kingpin 2speed