Most automobiles in the US are designed to carry up to five people, and almost always carry just a single person.

It still makes sense to target those with an MPG requirement.

We could also eliminate the Federal tax credits for farm vehicles -- actually we should alter the vehicles which qualify for the subsidy. Because current tax law encourages the purchase of vehicles larger than 8000 pounds for primarily personal, non-farm activity.

Almost insane touch that one, though; it hands the story to those who would oppose any reduction of carbon subsidy. I mention it by way of illustrating one of the myriad ways that hardworking, independent Americans depend upon Federal handouts. (Using emotionally provocative language on purpose here. I prefer to avoid such framing, not useful. Here I use it by way of another illustration of warped perspective on carbon economy status quo.)

What SUVs are carrying enough cargo to justify their mpg? I think commercial transport is of scope of what the commenter meant rather than the swath of empty pickups and jeeps being driven as daily drivers.