All the hoo-ha that the big three do not attempt to make more fuel efficient vehicles is mostly just that hoo-ha. They have attempted to make more fuel efficient vehicles with the base engines they use. Better induction and exhaust. Light weight materials. Two weeks ago I opened the hood of my wifes truck. I took a look at the intake. Traditionally, intakes were cast iron. Nice and heavy. Then they moved on to aluminum intakes. Very durable and half the weight of cast iron. My wifes truck has an intake that is plastic!
I thought WTH! Plastic on a hot motor? No wait, plastic can take the heat not to mention cold air flowing through it that keeps it cool. Also, probably the 1/4 of the weight of aluminum. So hey, why not? OK, they dumped quite a bit of weight here with the intake. Items like this were engineered and tested to work well and produce a lighter vehicle thus giving better gas mileage. However, these base motors are at the end of the line for improvements for mileage/gallon gain. So, it is on to hybrids. I'm glad to see Ford is leading the way for the big three!
I-25 has a point with diesels, these engines can be very efficient. It is just very hard to get people to swing over and make a change. This includes hybrids. One day there will be no choice. It is a hybrid car or you are walking.