jtr1962
Storage? I am Storage!
First off, read this. You're going on the incorrect assumption that car buying habits are going to remain the same as they were in the period from the early 1990s through about 2006. You're wrong. Trends come and go in any industry. The SUV was by and large a silly fad which is thankfully on its way out. Those who will continue to buy such vehicles will be mostly those who actually need them, as opposed to just want them. And with much smaller numbers being sold, they won't affect the CAFE as much as before.You can't break the laws of physics, so there isn't a lot that can be done to make the Suburban more efficient without making it a lot less useful...
Second, a lot can be done to make large vehicles more efficient without making them less useful. Vehicles use energy in two ways. One is the kinetic energy needed to accelerate up to speed. Usually this is wasted in heat in the brake pads when the vehicle must slow down. The second way is tire and aerodynamic losses. Let's start with number one. You can recover a good amount of kinetic energy via regenerative braking. Moreover, mass production means the components to do so don't add as much to the vehicle's cost. In fact, they save more money then they add in cost. As a bonus, an electric or hybrid electric drivetrain can offer fast acceleration without negatively affecting economy. The engine, if there is one, can be sized closer to average power demands, as opposed to peak power demands. That means less weight to lug around and fewer mechanical losses. You no longer have to choose between power or economy.
Now let's look at number two. SUVs are boxy and have high-rolling resistance tires. Neither of these things enhance their usefulness, yet both severely impact their fuel economy. Lower rolling resistance tires would help, especially at low speeds. Making the front ends more sloped and aerodynamic would help at high speeds, and wouldn't affect the vehicle's functionality a bit. You would still be able to load large cargo via the rear hatch, or pull a trailer if need be. Boxy is more a styling decision than a functional one. Besides making these vehicles IMO butt ugly, this decision also needlessly wastes tons of energy.
Finally, by making larger vehicles all electric, you can remove them from the CAFE numbers altogether. In fact, it's difficult to see how automakers can reach these goals without making a good part of the fleet straight electric. To me that's a good thing. With today's batteries gone are the days when electrics are glorified golf carts. My guess is once enough are out there, their advantages in terms of economy, acceleration, ease of use (i.e. home refueling), and reliability will be so great few will even want ICE cars any more. In 2020 someone may well be producing a movie called "Who Killed The Gas Car"! Moreover, future restrictions on auto use may well push manufacturers in that direction regardless. For example, it's easy to see large cities like New York passing laws prohibiting operation of non-zero emissions vehicles within their borders within ten years. IMO the future lies not with hybrids or even battery electrics, but rather with vehicles which get their power directly from the road, in a scheme like this. But it will take a while to put the needed infrastructure in place, so the aforementioned vehicles will be a stepping stone.