[eshteyn]

theoretically speaking 50% is the ideal number for just the motor itself, however depending on your drive train configuration, your gearing ratio, and your power delivery system the number varies. Also remember that this world isn't perfect and friction as well as inefficiency of design do exist and have to be taken into consideration. 75% sounds like the better percentage to achieve what you want, but theres nothing wrong with delivering a full 100% to the drive system, your batteries wont drain too much in the 2 and a half minutes your running them.