A Better Future

1. Plug in Vehicles
It is cheaper to buy electricity than gasoline per kilowatt-hour delivered to the wheels.  A Kwh of electricity costs about 10 cents and the charging and discharging in lithium batteries is about 75% efficient so one gets power to the wheels for about 13 cents per kwh.  Gasoline has about 30 kwh per gallon but even the Prius internal combustion engine is only 30% efficient so one gets 9 kwh per gallon to the wheels.  At $3 per gallon this means gasoline costs about 33 cents per kwh to the wheel.  In other words you can do 50 miles for $1.00 of electricity from a plug in source.
2. Efficient Gasoline Engines
Smaller engines are more efficient than larger ones, also new Variable Cylinder Management (VCM) technology further enhances the efficiency of engines by deactivating three of the engine’s six cylinders in a V6 during cruising and deceleration with no impact to vehicle performance.
3. Diesel Engines
Diesel engines, with computer assisted injection, use less fuel.  First the diesel engine is not a throttled engine and has far better efficiency under low load than a gasoline (Otto cycle) engine which is throttled under low load.  Second at full load the diesel engine is also more efficient as the compression ratio is more suited to recover the work out of the fuel.  At full load a diesel is about 38% efficient and a gasoline engine is about 29% efficient.  Diesel has a much higher specific gravity than gasoline hence there are more pounds per gallon and hence more BTUs per gallon in diesel than in gasoline.  Energy is also wasted creaking oil down to lower length carbon chains.
What a 4-stroke explained in more detail?
4. Duel Fuel
Imagine a car that handles like a BMW and does 80mpg.  For more details
5. Regenerative Braking
The energy that an automobile has when moving should not go to heating the breaking system, it should be stored and used again.
6. Lightweight materials
The less weight the vehicle is moving around the lower will  be the fuel consumption.
7. Capturing Heat from the exhaust
Work is under way in the UK at present to develop systems to utilize this energy.
8. Aerodynamic Shape
Regardless of what power source a car is using, the less its aerodynamic drag, the less energy will be needed to travel at any given speed.  The best shape is known as the tear drop shape...  Read on...
9. Better Wheels
Tweel  The tweel is a new wheel developed by Michelin that does not need air tires.  The wheel has urethane spokes that bend and return to their round shape.  The tire is simply a tread on the surface of the tweel and it can get retreaded over and over.

Examples that incorporate some of these ideas: Toyota Prius  Honda Insight Honda Accord

Nightmares

Hydrogen
None of the advocates of hydrogen as a fuel have done their homework.  This is not an opinion.  The basic laws of Physics are against it.  The main Achilles Heal of hydrogen as a fuel is storage in the vehicle.