Is there a rule of thumb for working out motor amp draw

[lincpimp]

As Arct1k said.

Please give us some info about your setup. You will also have to factor in that some batt mfgs overrate their packs, and a few other variables.

I can usually guess the necessary pack by the vehicle setup and tire size. Something like a monster truck with 6" tal tires and a properly sized motor that will turn around 35k rpm geared for 40-45mph will need around 2200 watts worth of battery power.