Most of our fuel is wasted in generting horsepower in overcoming aerodynamic drag.
Most cars today have a Cd*A of about 8.4, that is coefficient drag times frontal area, being about .35 and 24 square feet respecticely for a modeern car.
A 3,200 pound car with a Cd of .35 and cross sectional area of 24 feet square travelling at 65 mph will need about 33 horsepower out of its typical 160 horsepower engine yielding about 25 mpg.
Cut the Cd down to .2 and frontal area down to 22 feet square, and horsepower requirements drop to about 17 to maintain 65 mph. Half. Without changing anything in the engine, this will approxamately double highway fuel economy to 50 mpg, without cutting the car's weight or shrinking the car in size(smaller frontal area from smaller mirrors, perhaps). Basically, it is possible to have a 35-40 mpg V8 musclecar, by addressing aero drag, or a 50 mpg V6 full size family car, or a 70-90 mpg L3 diesel full size family car with average or better performance compared to cars currently available.
But less drag = less horsepower consumed at speed = less engine wear = less money for the auto monopoly pricks
I used 20% stray losses(transmission, wheel bearings, accessory loads, ect.) in this quick calculation, and assumed a constant tire rolling force of 32 pounds from a .010 coefficient rolling resistance of tires(in reality, rolling drag also varies with speed, but below 65 mph, rolling force would be lower than ~32 pounds, above, higher).
Of course, in 2050, if peak oil turns out to lead to a very severe doomer scenario, humanity might not be around to build cars by 2050, and if they are, new generations aren't likely to be learning all the engineering involved in making them.
If the crisis is more moderate and controlled, there will always be cars. They will probably be streamlined for low drag and fueled by batteries, or alternatively, very rugged to traverse broken down roads and diesel powered due to lack of refeuling infrastructure/electricity, depending on whether the transitioned scenario is closer to an Ecotopia/Technofix hybrid or Mad Max in its aftermath.
See the following topic for those interested in a discussion of automobile aerodynamics and the 157 mpg car subject of the topic:
http://www.peakoil.com/fortopic17785-0-asc-0.html
Personally, I think both Jato and Blistered Whippet both got the right idea as far as the more
likely scenario, despite what technology we've had for ages.
The unnecessary felling of a tree, perhaps the old growth of centuries, seems to me a crime little short of murder. ~Thomas Jefferson