The class of cars that got 10-15MPG now gets 15-20, the engines are cleaner, smoother, and more reliable. Less regular maintenance is required due to the lack of points, choke adjustments, introduction of fuel injection (electronic), and long life spark plugs.
When you compare power you must compensate for the gross vs. net ratings. 400HP now is alot more than 400HP in the 60's.
Part of the reason why gas milage hasn't really improved like it should have is that cars have gotten heavier (not lighter). Although todays cars have light weight composite materials, those materials are packed in tightly in the form of safety equipment, emmisions equipment, accessories, and insulation.
When you see that 4000Lb chevelle rolling down the road, look at the 4000LB SC430.