A few months back, longtime commenter Mattias (of Regards, Mattias fame) hipped us to Alpina's D3, the venerable BMW collaborator's take on the BMW 320d. Now, the D3's on the radar of Green Car Congress, which lauds the D3's high mileage and low C02 rating and Euro-4 compliance via particulate filter. Alpina applies a larger Garrett turbocharger and intercooler, larger injectors and tweaked engine management software to the stock model, boosting horsepower to 200 and torque to 302 lbs-ft, while retaining mileage of around 40 mpg. It's not the quickest model around (zero-to-60 in 7.4 seconds) and it wouldn't likely make it past the emissions police in California, New York and Massachusetts, but with the mileage figures in the Prius range, it most definitely can serve as a discussion opener.
BMW Alpina 2.0-liter Diesel D3 Delivers 100 HP/Liter and 40 MPG US [Green Car Congress]
BMW Alpina B7 to Arrive in US [internal]