My 8,000 pound Dodge pickup, with a diesel engine, gets 20+ mpg if I drive the speed limit. If it can do that why can't a 4,000 pound car get 40 mpg or a 2000 pound car get 80 mpg? Easy to answer, EPA required smog equipment. Clean air is nice and I like it. But they have gone far beyond the point of diminishing returns to the point that the exhaust on most cars is required to be cleaner than the air most of them take in to burn. That's stupidity of not just the highest level but even above that, it's of the government level. They forbid modifications or systems that would actually improve mileage and air quality because they aren't "stock".