Germany ISN'T the US. If the price of gas increased by o' say double, the US would change quite a bit of it's behaviour. And since we are so "bad" that would probably solve the problem completely.
The cars in germany/france are already much smaller than the size of the cars in the US. So we basically cut the size of the cars to the size in europe but that's not enough for you? I don't get it, it would do the same thing as raising car mileage by governement fiat, but you don't like that because it's not by government fiat?