As someone that has spent some time in Europe, and being American in the land of Metric, I just found myself converting in my head, even though I was looking at say a speed limit of 120kph, I did the quick and dirty math of 62.5mph+13(ish)mph=75mph... not perfect, but close enough to be wary of tickets. With other household type measurements, it was a similar situation. A touch less than 4 liters made a gallon (3.7 as memory serves), so when I saw prices per liter, I'd just average that times 4, and have an idea of the prices in what I was used to.
I'm not saying, by ANY means that I'm some sort of math whiz, but I'm not sure the majority of Americans would put in the little bit of time it takes to work out these simple conversions if we were to go metric. I know I wouldn't mind a bit, even though in my head, I'd still be thinking of "75mph" instead of "120kph", lol.