I fully agree... I am of the old school, KISS (Keep It Simple Stupid). Most of the big box stores know less about the cable you are buying than you know. Also, there is no reason that the car should be more or less efficient at different charge rates as long as the Cable & Circuit were designed to handle the load. I can see if you had an undersized cable and attempted to draw a heavier load than the cable could carry that you might into an efficiency issue.