Why don't engineers and GM themselves think about this though..
If the tank is smaller, sure the weight is less but it will also pollute more because if you have to stop for fuel more often you are using more fuel to reach the station, sit at stop lights and merge back into traffic.
Some of the stuff the EPA and CAFE do is just stupid because all the stuff they do doesn't exactly make things better. It costs more to produce these vehicles, the environmental impact greater when it comes to mining for materials. Diesel engines require that low sulfur fuel which takes more time/chemicals to make, they use DEF fluid which is another thing going into the air and when a regen happens guess what it uses even more fuel..
But yeah, whatever EPA. Keep doing you, keep being a bunch of hypocrites.