Behind the Design has a good documentary on the relationship of torque on the motor and the current it draws.
http://frcdesigns.com/2013/06/24/beh...earbox-design/
With a CIM having:
a free spin of 5310 RPM
free current draw of: 2.7 A
Stall Torque of: 343.4 oz-in
and Stall current of 133 A
You can work the equations listed in the document for how much torque would be available for 20A (or another number, that's just what my team has used), and use that torque and a gear ratio to find available torque at the wheels when only pulling 20A. If you can move at this current, use the gear ratio again to determine what torque at the wheels would stall the motors (this can be combined with the wheel size to get a max pushing for - ignoring weight of the robot).
I wrote a python script that can calculate the gear ratio to move a desired speed and the minimum number of motors to move at that gear ratio assuming a coefficient of static friction of 1 (static friction must be overcome to get the robot moving). I can post it if desired.