It's a classic physics problem with many variables, perhaps too many to model mathematically for an easy answer.

I would suggest having the assembled tanks on the work bench or better yet the terrain type (like dirt) it will be run on and attach a torque wrench to the sprocket an measure the static force and the force needed to get it moving (one track only to simulate power for turns). The moving force will be less than the static force.

A torque test on maximum incline (vertical lifting 20lbs.) would also be advisable to determine the max motor stall current it may experience.