There are (at least) five or six things that automobile makers could do to improve the effiency of all cars/vehicles:
Using the variable valve technology that is already in many engines, to not only improve the power band (less air flow at low RPM's for more torque, and more air flow at higher RPM's for more horsepower) -- but to also use it to shut off cylinders, when they are not needed. A few V8 engines already do this, and transform themselves in to 4 cylinder engines, when the extra power is not needed. So, why not turn a V6 into a 3-cylinder, or a 4-cylinder into a 2-cylinder? Seems to make a lot of sense to me!
- The Toyota Prius (2nd gen?) uses a vacuum-insulated tank (like a Thermos) to hold hot engine coolant overnight, to be pumped into the engine block to allow it to come up to a good operating temperature much more quickly. This could/should be done on all vehicles, and it would greatly reduce emissions and fuel use at the start of every trip.
- Preheat the fuel right before injecting it into the cylinders -- this will vaporize the fuel (as opposed to simply making it into droplets). This will help increase the power and efficiency of any engine dramatically!
- Use a CVT (continuously variable transmission) instead of a automatic transmission with a torque converter. A CVT transmission provides nearly zero slippage, and it helps the engine operate at the RPM that is best for the situation, and the ratios are infinitely variable, and there is no lag time while you shift.
- Using a CVT transmission makes it possible to do something that the Honda Civic Hybrid (and others) does, and that is shut off the engine when the vehicle is stopped. The instant the driver removes their foot from the brake, the engine is started and it goes on it way, without you even being aware that it was not running.
- Work on reducing both rolling resistant, and aerodynamic drag -- both the underpan of the vehicle and the body, too. Tandem with this, there should be an alarm for when the tire pressure drops below optimum.
Can you think of other improvements?