[steveaca]

I think that your motor's torque (216 gm.cm.) is markedly less than the other two motors mentioned and for rallying, acceleration would be more important than top speed, a lower gear ratio (numerically higher) would be preferable. In this case, 3.875:1 (31/8) would be preferable to 3.000:1 (27/9). That's a big difference however, and you could possibly go for something in between, say 3.500:1 (28/8).