[BrianG]

First of all, the overall power a system will draw is determined by vehicle weight, geared speed, terrain, drive style, motor efficiency, etc. Then, you use ohm's law to calc current based on the power needs and voltage used.

There is no way to exactly calculate amp draw since there are so many variables, but you can get fairly close. The best way is to use a data logger, but failing that, estimating based on similar setups can get you in the ballpark. The "car/truck setup guide" and similar tools (link in my sig) can give you a starting point.

Also, be careful with amp specs on ESC's and motors. Some may be inflated to make them seem better than they are in reality. And even so, the load on the motor determines the current a motor will pull, and therefore what an ESC has to handle.