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Can't speak for today with the computer controlled engines. Technology has improved a lot. In 74 I bought a new Nova 0 miles. Right out of the crate, mileage was around 12. 350 4V automatic with 3.07 gears. Changed the exhaust to headers with duals milage went to a strong 14. Changed out the cam and intake for better breathing, milage went to 17-18. After tweaking the carb (spread bore Rochester quadrajet), balancing and blueprinting the distributer (point and condensor shoved the timing from 4 BTC to around 16 initial around 32 total) and air filter, changing rear tires to 15 from 14 inch. Mileage leveled out at about 20-21 keeping my foot out of it. Put your foot into you just hung on for the fun ride on the way to the tire shop!!!!! !!!!! Pay off for all of the above in addition to intial cost (about $500 or so, i did all my own labor) was tune ups were done every 3500 miles. That's all the plugs could handle. Today's engines have to "go" a certain milage without tuneups and have been "de tuned" to achieve that. The very first place to start with any engine is to get it to breath better. On BOTH ends. Today's diesels already run the turbo for most of them so the input is somewhat taken care of, but for gassers fresh air intake improvements and exhaust will give you the best bang for your buck.
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