Here's part of an article from Ecogeek. To read the full piece, go here.
VW has gone ahead and approved its Jetta diesel hybrid concept, hoping to turn the car into a legitimate Prius competitor. The car will likely be roomier and zippier than the Prius, both advantages coming from its use of diesel instead of gasoline.
Diesel, which contains more energy per gallon and also produces less CO2, can improve mileage significantly on its own. Adding a hybrid component to the car really moves up mileage numbers. Unfortunately, diesels require advanced and expensive engines to keep pollutant emissions low. VW already charges $2,000 extra for its diesel models. Add onto that the cost of regenerative braking and a hybrid drive train and suddenly this car is looking out of range.