Here in Canada, my family ran a fishing camp with a lot of US customers. I was talking with one of them about metric vs. US units and to demonstrate I said how about this: How many gallons in a cubic mile? And he was able to work it out in his head, and pretty quickly too. Whoa. The point I was trying to make that is than in metric (say, liters per cubic kilometer) you just need to get the number of zeroes right, but you can't argue with results.