Hi friends,
We have been developing an autonomous Rover to run in an agricultural lands. We are using the skid steering by moves the front and rear wheels of left side to forward and front and rear wheels of the right side to move backward.
Weight of the rover system: 180kg
No. of wheels used: 4.
No. of motors used: 4 (for four wheels to four motors by chain sprocket) or 2 motors (by 1 one of the Motor shaft is coupled to two wheels by chain sprocket)
Wheel radius: 12 cm
Track width: 52 cm
For that we need to choose suitable motors of required torque. So I kindly request the community members to help with the torque calculation to how much torque does each motor need to run this system. If any information need for torque calculation im happy to provide those. Im sharing our system with this post. Looking for the members to respond for this topic. Also kindly share the post with your friends and share the calculation here please 
The basic formula for calculating the torque is M = F x r, - i think you know… 
To calculate which moment you need, it needs a lot more data like: friction, at which type of ground, gradient to go, required acceleration…
Btw.:
It seams i’m working at a similar project, - a rover/crawler for off road use. In my mind your wheels don’t have enough with for that ground…