DC Cable Sizing Tool - Wire Size Calculator - MM2 & AWG - solar-wind.co.uk
That is the cable sizing calculator that I used, with 5 gauge (what the calculator gave me) at 400 amps, 72 volts will drop to just under 70v if you have 3% loss in your cables, so of the 28,800 watts the batteries would be putting out at that point in time, 800 of those watts would be turned to heat, once you hit cruising speed and are drawing 100 amps or so you could have 6 gauge wire and only loose 1%, so while I agree with Ryan that larger cable is better, there is something to be said for spending less, having less weight and having the cable take up less space and being easier to route, that is why my Commuti-car (same make as Ryan's) has been upgraded from the factory 4 gauge to 2/0 cables, of course it draws near 600 amps peek and has over 25 feet of battery cable.