I vaguely remembered from my previous Physics class the "Input Equals Output" which amounted to 100% efficiency. But the 100% effici3ncy is not always achievable so the Output could be somewhat less. Does this theory or accepted fact still hold true in today's advanced technology? For example: My 2000 Acura 3.2 TL is advertised to have 21MPG City fuel efficiency. Today Acura is boasting a mileage of around 30MPG. How do car manufacturers achieve such fuel efficiency? I suspect that something else in the automobile such as weight or performance and Return On Investment had to be considered. What say you?

There are loads of changes in 14 years. The fuel distribution technology in your 2000 is vastly different than today, which is direct injection, like a diesel. Is the engine displacement the same? Your 2000 probably has a 4 speed, my 2006 Nissan has a six speed and at 70MPH is only turning 2000 RPM. I had a 1957 TR3 with a 4 speed that at 70 was turning 3500 RPM. Some newer models have 10 speed. Ford is going to Aluminum body panels to reduce weight. Aerodynamics plays a part. I would say weight, fuel injection and RPM are the 3 big factors. Even with all those changes, the efficiency of a gas engine is probably less than 20-25%.

Indeed it is still true - the idea that output could be greater than input violates the 2nd law of thermodynamcs, and would allow the development of a perpetual motion machine.

In addition to the technologies that ma0641 mentioned I would add that modern electronic engine management systems and sensors are much more sophisticated now, having the ability to change and optimize the ignition timing and air/fuel mixture constantly depending on variables such as air temp, exhaust mixture, engine load, and RPM.