Jimmy Carter had the mileage increased on cars and trucks over a phased period of time. It was enough to eliminate imported oil once all cars got the mpg up. Well, they didn't take into account several things happening.
One, there were 200,000,000 people in the US in the late 70's. Today there are over 300,000,000. We had about 100,000,000 cars and trucks in the late 70's. Today we have 175,000,000 cars and trucks.
Then, when cars got smaller, people went to mini-vans and SUV's if they had families to have room to transport their kids and stuff around. Mini-vans and SUV's were built on truck frames so the mpg requirements were less on trucks than cars.
Then no-one wanted a refinery in their backyards, so no new refineries have been built since the 70's.
So, we are still were we started. Changing to natural gas and diesel would cut most imports. Also, some people need gas guzzlers for their work trucks, or they have large families. They also may have a boat or camper to tow, and need a larger vehicle. Thus they may need a gas guzzler. This is America, if I want a gas guzzler, I will buy a gas guzzler.
Insurance is less on a big vehicle. We traded in a Ford Escape SUV for a Ford Expedition. Our insurance dropped $100 a month. I have a boat and a trailer to tow.