[suicideneil]

4 A123 cells = not enough voltage/power.

With 6s A123 cells, I would gear around 68/18 for ~40mph (new 3905 emaxx, stock sized wheels).

Thats plenty of speed, but remeber that 6s2p ( 2x 6s in parallel) is much better and safer for the batts and esc compared to just 6s.