I keep saying over and over, the way you reduce the consumption of something is to tax it, not micromanage design.
Consumers and industry would come up with an optimal solution if petroleum were taxed higher, just as European consumers have chosen more fuel efficient options. It's not that Europeans are genetically superior or something, it's simply that they respond to incentives and disincentives just like anyone else.
If we were serious about reducing fossil fuel consumption, we'd simply progressively raise taxes on it. Then instead of a whole bureaucracy required to police the industry to see which companies need to buy carbon credits, you could replace it all with a single job that evaluates if we're on track to meet consumption goals and adjust as necessary.
As an aside, vehicles have become more fuel efficient over time despite their size and weight increases.
