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In the old days of muscle cars cars burnt lots of extra fuel. The extra fuel cooled the engine and allowed the engine to run at its most powerful fuel-air ratio. They got terrible mileage. Now with computer controlled engines, and also much better engine designs we can have efficiency and power at the same time. I read somewhere that a GM V-8 of the 1960's cost GM $25 to build. In todays dollars that would be maybe $200. I suspect that our much more complex engines of today cost more.
Variable valve timing, which flattens the torque curve, allows efficient operation at low RPM, which reduces engine friction and pumping losses. Yet these same engines when revved to 6000 or more RPM produce gobs of power. So a modern engine with a six speed transmission can drive a car at 80 MPH at less than 2000 RPM, and by shifting down and revving up can deliver 250-300 HP with no efficiency penalty. Small cars of the '80's weighed around 2000 lbs; they now weigh over 3000, except for the very small cars now on the market. The engines of the'80's would get terrible mileage pushing todays (relative) behemoths.
Those who are old enough can remember the Messerschmitts and BMW Isettas of the '50's. If they were out today with modern engine technology they would probably get 80 MPG in the city. And if you got hit by a modern sub-compact you would die.
The Insurance Institute of Highway Safety has a web site that charts the death rate per million registered vehicles of almost all cars on the road. My Odyssey has one third of the fatality rate of a Suburban, for example, so size isn't everything. But a car can get so small that it is inherently hazardous to drive it. So we will have fairly heavy cars for the foreseeable future , but modern engine-transmission combinations and hybrid technology will sure help. I think we will have $10 gas within five years. Make your plans accordingly.