I just read with increasing interest this thread, it is relevant (we all want better mileage) and timely. Just want to clarify, if possible, some points made by all.
First off, Craig's statement of thermodynamic efficiecy is right on the money. It does not matter what engine, cycle etc you use, efficiency only depends on maximum and minimum temperatures achieved in the engine. By the way, the maximum temp is not the coolant temp but the combustion temperature which is much higher, depends on the type of fuel and fuel/air ratio among other things.
Second, Rick & Connie are right in that science, by definition, is fallible and imperfect and will change with time. It is possible that in the near future all this knowledge is proven wrong. If you are a scientist you have to keep an open mind and be prepared to accept new improved science. Having said that, however, I must also say that you have a much better chance of winning the lottery in all 50 states in the next couple of years than finding, in the next couple of years, that these basic laws are wrong.
Third, there is a piece of the puzzle missing. Engine efficiency is only part of the equation. A car stopped at a red light has zero efficiency as a transportation system. The car needs to move to have a propulsive efficiency above zero. The overall efficiency is the product of the thermal efficiency and the propulsive efficiency. Propulsive efficiency is lower than one and changes from car to car, that is why the same engine on a car and a truck would give different mileages. Hybrids do not alter either efficiency directly but rather recover and store kynetic energy that would be lost when braking for later use. Last generation hybrids however have been designed to increase low rpm acceleration rather that improve mileage.
Improving mileage is an extremely complicated process involving a great number of factors both mechanical and human. I am convinced that there are no magic solutions. I guess I'll play the lotto.
That's my 1/2 cent.

JL