There is a breakdown I found
for where the energy in a vehical goes:
Start with 100% energy in the form of fuel and put it in your engine
62.4% goes to "Engine losses"
17.2% goes to "standby/idle"
and 18.2% turns the driveshaft
This 18.2% is broken down to
2.2% for "accessories"
5.6% for "driveline losses"
and 12.6% turns the wheels
This 12.6% is broken down to
2.6% "Aerodynamic Drag"
4.2% "Rolling Resistance"
5.8% "Inertia/breaking" (which is the real moving of the vehical)
Based on this, the greatest improvements can be made in the engine. Looking at vehicals with things like drag and rolling friction being the same (similar sized and shaped vehicals with the same type of transmission), I still see quite a variation in mileage. Going hybrid helps with that 17.2% that is for standby/idle and only really affects the city driving mileage so this more then doubling of mileage some cars achive must be better engines.
I am quite surprized how little this breakdown asigns to Aerodynamic Drag and Rolling Resistance but then again my shcool bus with it's duel tires and large non-aerodynamic front gets better mileage then some of my sleek cars.
This shows that the real key to better fuel economy is a new type of engine.
Some possible alternatives are Quasiturbine engines. They are much like those wankle engines but seem to avoid lots of the sealing problems Wankle engines have. The lost energy from engines turns into these energy forms:
unburned or incompletely burned fuel
hot exhaust gasses
so reducing these things will increase mileage. Lots of people with Honda Civics complain that they never get hot enough to defrost their windows in the winter. These cars also have the best non-hybrid mileage... I think I see a connection.
Noise and vibration is a hard one to go by because many of the heavy gas guzzling engines are quiet but that is because all the vibrations just go to heating the coolant in the engine.
Emission standards are forcing vehical manufacturers to reduce this part of the losses BUT things like Catalytic Converters are no improvement as far as mileage because all they do is burn the unburned gasses before sending the hotter exhaust to the muffler.
Turbochargers and preheaters help keep the energy (heat and pressure) inside the engine and not as much to go down the exhaust pipe.
Still working with a normal engine, here are some things I found to keep my mileage at the highest possible:
1) proper timing and proper gaps on spark plugs. - they say diesel engines are more efficient (30-40%) but that might be partially due to bad timing and gap on most gas engines.
2) use the right fuel - Engine designers make the flame front inside the cylenders to work best with a certain octane level. Find out what the right octane level is for your engine and use that... even if it is more expensive per volume, it is usually cheaper in terms of cost per mile driven.
3) make sure your filters are the best possible and clean (fuel, air, and oil)
4) lubricants can make a big difference. There are a number of lubricants that coat the inside of your engine and I find them very good at improving mileage especially during cold weather.
5) lots of engines have fancy detectors and vaccuum hose adjustments. These things make a big difference if they are in order and adjusted right. Most are easy to test and fix using a repair guide book and the others can be optimized at the dealership.