VW has announced they are going to make the XL1 plug-in hybrid. They are only making a small number of vehicles and it sounds like they will be close to handmade, but the tech is impressive. The IC engine is as close to ideal as is possible for the moment- a small turbo diesel mated to a 7 speed dual clutch transmission. It is sized for low accelerations (0-60 is about 12 seconds) and generally what you would want for economy. The car is very low weight (795 kg) and has a very low drag coefficient and frontal area for a car.
The engine can drive the car, but there is also a plug-in hybrid mode with a small (5.5 kWh) battery good for about 100km of electric-only travel. Regenerative braking of course and many other details.
Low hand-built production means we'll probably never see one in the wild (a batch of 50 has been mentioned), but the power train is one of the most interesting pieces and apparently it will be used in a real production VW - fantastic news for anyone (probaby in Europe though) who wants an extremely efficient car with a small enough battery pack that the price may not be as high as other plug-in hybrids. It may be an ideal car for city and suburban travel - low accelerations, but lots of them and relatively low total mileage for trips.
Autocar is reporting the power train will go into a version of the Up! - a car that has been judged too small for Americans, but already very efficient with conventional IC power
plants and just crying out for something like this. For an idea of scale, the UP!, even with the battery and new powerplant, should still be under 1000kg.