Diesel are only efficient for heavy cars!

I think there is quite some misconception. A Diesel engine is only - and only fuel efficient in heavy cars. Efficiency then is really realtive. Since heavy, they are still environmentally questionalble. A Diesel engine is a complex and heavy engine, that does not pay off for lighter or even smaller cars or mid-size cars.
So here is an example, take a typical German Family car such as the OPEL Zafira (which not really is a small car) and go to the OPEL site and check out if the Diesel is really more efficient than the gasoline engine. You will find, that it is absolutely not!!!! The Diesel produces exactly the same amount of CO2 per kilometer and PS like the equivalent gasoline car! Contrary, the current Zafira model sold in Europe is not even a clean Diesel yet. If it were coming with a good filter, it would actually produce more CO2 per kilometer! Check it out yourself:
http://www.opel.de/shop/cars/zafiranew/product/engine/content.act