You could just make them smaller. There's no rule saying you can't.
There kind of is. Fuel efficiency requirements are stricter for smaller vehicles, creating a perverse incentive to increase size. Here's a 2011 paper that predicted exactly what would happen: https://news.umich.edu/cafe-standards-create-profit-incentiv...
There kind of is. Fuel efficiency requirements are stricter for smaller vehicles, creating a perverse incentive to increase size. Here's a 2011 paper that predicted exactly what would happen: https://news.umich.edu/cafe-standards-create-profit-incentiv...