I want to try to list steps that car makers could/should take to improve the fuel efficiency, in approximate order of cost:
* The most obvious would be to mold the plastic on the front of the vehicle to be smooth and round, with grill openings that are sized (and placed) only as required to cool the engine. Some new plastic bumper covers and grill pieces could be snapped onto cars. Flush covers should be used on all lights. Fairing pieces could be added to side mirrors. "Zigzag" window trim covers (would smooth out turbulence), and hood gap gaskets, and streamlined wheel covers are easy, too.
All these improvements (above) can add at least 10% and as much as 25% to the fuel economy. Here's more:
* Sealing all the seams and joints (with gaskets or backing flanges) in the high pressure areas of the vehicle.
* Smooth floor pans with no sharp protrusions -- this is part of the drag just as much as the parts of the car that can be easily seen.
* Transmission ratios should be optimized for efficiency at normal driving speeds. Higher gearing with 6-7 speeds would be optimal (smaller engines need more gears to work efficiently); or CVT transmissions.
* Narrower and lighter, LRR tires should be used.
* Average & "instant" MPG displays, that are on all the time. (The Scion xD uses the same display for the odometer, the two trip odometers, average MPG, instant MPG -- way too many things on one display! You have to toggle through them with a button that could not be a bigger pain to press...)
* Throttle control (rather than cruise/speed control!) that maintains even throttle in a user-set speed range, with minimal throttle increases to maintain the speed in that range.
* Tire pressure monitor, that warns the driver when the tires get below the recommended pressure (which could be adjusted upwards if the driver wants to run a pressure up to the tire sidewall maximum).
* Efficient and effective fresh air flow through the vehicle, with intakes on a high pressure area, and exhaust vent(s) out the back of the vehicle into the low pressure wake zone, to reduce drag as much as possible.
* On defrost mode, the A/C should only come on automatically at maximum defrost(if at all), and it should always be toggled on/off by the driver. I would prefer to make A/C on the defrost setting optional.
* Fully ducted engine cooling system, with the intake down low on the front, and the exhaust flows into a low pressure zone; minimizing the size of the radiator and the reducing the "internal" drag as much as possible.
* Video cameras and screens in place of side mirrors.
* Automatic engine shut off and start up; at least with CVT (or automatic) transmissions -- and possibly with standard shifts, based on having both the clutch and the brake applied?
* Lean burn in low vacuum conditions.
* Store hot coolant in a vacuum insulated tank, to speed up warm-up time; a-la what the Prius does.
* Coat all the window glass to exclude as much heat as possible.
* Tighten up wheel openings, and always use aerodynamically designed wheels/covers, with rear wheel skirts (at least optional).
* Make roof racks removable. Years ago, I saw a "papoose" add-on storage system that locked onto the back of the car, with a single caster wheel to support the weight -- it tucked completely into the air flow behind the car; and it would be a great way to add storage space when needed; that did not affect how you drove very much. It could actually greatly improve the overall aerodynamic drag of the vehicle.
* For new 4-cylinder engine designs, the crankshaft could be split with a hydraulic coupling that can automatically disengage two cylinders completely; saving all the pumping and friction losses, for situations when 2 cylinders are enough to provide the required torque to move the vehicle.
* Nissan is (supposedly) going to reduce their cars weight by an average of 15%. I think all cars could be reduced by 20-30% with smarter steel fabrications, smarter use of materials. Here's a site that shows a steel chassis that is 25% lighter and nearly twice as stiff/strong as a conventional steel chassis:
* * Or, make every vehicle with a plug-in electric w/ serial hybrid ICE drive train, with regenerative braking.