Those cars from the 1980s will never:
- meet today's crashworthiness requirements (engineered crumple zones, safety cage, etc).
- meet today's safety equipment standards (airbags, antilock brakes, electronic stability control, etc.)
- meet today's emissions standards (things engineered into the engine and the exhaust system).
Put all those things on your 1980 Duster, it will gain about 600 pounds of weight and it will never get anywhere near 40mpg.
Get rid of all those things on that 2005 Honda Civic the above poster mentioned, it will lose about 600 pounds of weight and it will get 60+ mpg.
There you have it. Over the years, cars have gotten heavier because of the engineering and additional added equipment, and bigger engines to push it all. That is why you can't really make a fair comparison between the MPGs of 20-year-old jalopies vs. cars of today.
Frankly, I'd much prefer driving a more current car for the added safety equipment and crashworthiness despite the decrease in fuel economy.