Thereoretically nuz69 is right. The setup only draws as much power as it needs for a certain speed. If the output power required to go 40Mph is 2500W, then for the same vehice it will ALWAYS be 2500W to go 40Mph. What changes however is the input power from the battery based on the efficiency of the system.

nuz69's theory is based on the assumption that the efficiency of the 1518 will be the same as that of the 1515, with a 4S lipo pack. This is incorrect as the 1515 will have the most efficient top RPM of 32,560 whilst the 1518 will have a max RPM of 26,640. At 26,640 the motor is in a lower portion of its power curve, and is not as efficient compared to an RPM of around 30,000. The 1518 motor is more efficient when run on higher voltage, in the right RPM range, compared to the 1515 on a lower voltage - as it is high current that increases the power loss in the system.

As a result, the lower efficiency of the motor will demand more INPUT power from the batteries. More Input power for the same voltage means a Higher current draw. Therefore, running the 1518 on 4s will draw more current from the battery resulting in shorter run times. Not to mention higher temperatures.

Xsmoker, I think there are only two options for you. higher capacity 4S batteries for longer run times or a 5S setup with a 1518 and lower gearing. Cheapest would be the larger 4S batteries, since the other option requires you to buy a whole set of new 5S batteries and a 1518 motor.