Cars are more efficient today. The prius can beat those numbers (51/48/50 mpg hwy/city/combined) even with it weighing 700 lbs more than the 85 corolla. (2300 and 3000 manufacturer listed curb weights)
The first gen Honda insight is listed at 500 lbs less than the 85 corolla, but was rated for 70 mpg in 1999. (EPA)
They did limit speed limits before. It didn't do anything. The DOT conducted studies saying it only helped reduce gasoline consumption by 2% , other studies showed even less. (Wikipedia link:
National Maximum Speed Law - Wikipedia, the free encyclopedia )
That said, I agree with you. If we were all as crazy about saving fuel as some of us are here, it would not be a problem. But a problem-less world would just be so boring, ya know?