Unless I'm mistaken, Drive Calc is mainly intended for aircraft. Of course, a motor is a motor, but aircraft tend to be much more predictable because the current draw is more even with very few bursts. When dealing with land-based vehicles, there are a LOT of variables to consider; drive style, terrain, vehicle weight, tire style, etc, etc. And cars are much more "peaky" for current draw.

The easiest way to get a rough approximation is to run whatever battery you have right now while timing the run, and use this (http://www.scriptasylum.com/rc_speed/power.html) to re-figure it using different battery configurations.

For instance, if you get 20 minutes of runtime on a 6s 5000mAh battery, your estimated power usage is 15A average with 90A peaks according to the link above. If you go down to a 6s 3000mAh battery, the runtime will be reduced to 12 minutes, and you would need to make sure the battery has a true C rate of 30C or more for those 90A estimated bursts. Note that since the 3000mAh pack weighs less than the 5000mAh pack, your runtime will be slightly higher than 12 minutes, but it at least gives you a rough idea.