No, definitely not. Some of the older cars were far more efficient than any of the newer ones. The old Geo Metro is a perfect example. It pushed 50+mpg and had an extremely simple 3-cylinder engine in it. The newer version now badged as the Chevrolet Aveo weighs more, has a larger engine, and gets worse mileage but is essentially the same car.
Another fine example are the old smokey VW Diesel Rabbits. They still get 50-60mpg from a normally aspirated diesel engine. The exhaust is not as pleasant as the newer high-tech diesels, but the cars didn't have to push the exhaust through all the smog devices and thus are much more efficient.
Also consider how much pollution your creating by buying anything new. All those materials in the new cars had to be produced somewhere - and production produces about 10 times more waste the the product itself.