Sorted (gotta say it with a northern English accent- the 't' is kinda silent)...
66/18 = 35mph
66/20 = 39mph
66/22 = 43mph
66/24 = 46mph
66/26 = 50mph
66/28 = 53mph
KershawDesigns for oversized pinions. I'd say gear it for about 40-45mph tops really, though it mainly depends on if the tires can stay in one piece, and what the esc/motor/lipo temps reach.