Here’s a chart that appears in a (less-than-groundbreaking) discussion about peak vehicle miles traveled, showing average US fuel economy since the 1970s.
According to the page (Peak Oil News):
The graph also shows that, with the second oil crisis, mileage restarted to increase, but by far not as fast as in the 1970s. There is a reason: it is difficult to optimize something already optimized. This we call ‘diminishing returns of technological progress.”
Not sure I agree! Where do you see fuel economy headed in the coming years?