Ok, I'll admit this is a crazy idea, not likely to ever see the light of day, but it's worth throwing out as an exercise. It is often said we should keep the speed limit low to encourage good fuel economy.
What if there were no speed limit, but instead a "fuel limit." For example, 2 gallons of gas per hour.
If you are driving a 30mpg Honda Accord, you
could thus go 60 miles an hour, as that would burn 2 gallons in one
hour. If you had a 50mpg Toyata Prius, you could go 100 miles/hour
because that would also burn 2 gallons/hour. ZEV? As fast as you
want, just like Germany.
Bad news in that 20mpg Ford Explorer 4WD. 40 miles/hour maximum speed
A Hummer? Forget it, you'll be below the minimum speed. No highway
driving for you. Enjoy the side roads.
This is more an academic exercise, as the voters would reject it (they
love their big cars) and we actually don't want anybody going 40mph on
I-280. But in fact, if the limit is set to encourage good fuel economy,
this would be the way to do it. A simpler, but also unpopular way
would be to tax gas to $5/gallon. It's fair, because when you crud up
my air by burning your gas, you owe me something, and the least you could
do is reduce my taxes.
Now of course if you really did something like this it would be more complex. Probably not linear, nobody below 50mph. If 50mph is not enough, an 55mph exception for commercial trucks, busses and other non-passenger or many-passenger vehicles. Being a carpool would also alter your speed. Of course it would be a fair bit harder for cops to enforce, but actually not that hard. Cops are trained to identify cars very quickly, and of course would have a quick database. But frankly, you can tell a guzzler when you see one, most of the time.