We have a 1st generation HL Tiger that was upgraded to smoke and sound electronics (TK-RX13 board) but still runs plastic tracks and gearboxes. It did 2.49mph or 4.01Kmh on a speed run with the black capped motors, mathematically working out to about 15,200rpms under load.

Scale speed for the Tiger is about 1.50mph/2.41Kmh.

The white capped motors running through metal gearboxes and tracks ran about 1.56mph on another Tiger, about 9500rpms.

I don't know the differences between boards, if there are any as it relates to these tests, as TK-RX13s are still used in our motor pool.

Judging from the available data, the black capped motors are fastest and should work well with your TK-RX18.