Fair enough R.P.L., but how would we know that the guy who said that Americans drive with the left foot on the brake actually meant that some of them may drive with their left foot hovering near the brake pedal?
He said that they drive with one foot on the brake, if this was true, then their brake lights WOULD be on, and their fuel consumption would be extremely high, as you quite rightly pointed out.
I don't know, but it seems completely crazy that a large number of people would do that. Having lived (and driven) in the US recently, I can also confirm that it's definitely not the case anyway. So I based my assumption on those two points.