When Mercedes Benz first introduced Bluetec clean diesel technology in the 2007 E320 Bluetec sedan, many saw the comeback of diesel in the United States. The idea of a luxury car that gets better than 40 miles to the gallon—with particulate filters and after-treatment devices to minimize diesel emissions—sounded like a winner. But in the end, those emissions were only clean enough for 43 states.

Since then, Benz engineers incorporated a liquid called AdBlue into the exhaust system that further scrubs NOx particles. The new and improved Bluetec system is now 50-state legal and will be featured this fall in a trio of high-end Mercedes SUVs: the R, ML, and GL. We had an opportunity to drive one of these SUVs, the Mercedes R320 Bluetec, to evaluate its efficiency. The results fell short of our expectations.

Our first driving loop was a highway route that took us westbound from Baltimore to Cumberland, Maryland—a 142-mile stretch on Interstate 70. The road posts a 65 miles per hour speed limit, though most traffic was moving about 80 miles per hour. This test route yielded just 18.5 miles per gallon. The EPA rates the vehicle at 15 in the city and 23 on the highway, so we were expecting something, at least, in the low 20s. Heading back to Baltimore on the same route confirmed our findings with a calculation of 19.2 miles per gallon. More disappointment.

Our second test loop consisted of urban-suburban driving in and around the Washington, DC metropolitan area. The highest speed traveled was 40 miles per hour, and there was considerable stop-and-go traffic. Once again, the Bluetec SUV fell short. Where its sedan counterpart two years ago managed stellar numbers in city driving, the Mercedes R320 yielded just 14.4 miles per gallon on a 110-mile test drive.

Mercedes introduced its Bluetec vehicles to most of the country two years ago, and is now prepared to debut the cleaner diesel offerings on a national stage. But even before the curtain rises, there may be trouble backstage. The current price of diesel is about 20 percent higher than gasoline. That will keep some customers away. In addition, Mercedes is applying the diesel technology to luxury SUVs, in a time when the economy is struggling and larger vehicles have fallen out of favor. With the disappointing fuel economy numbers we experienced, the argument for diesel in America may be further weakened upon release of Benz’s newest Bluetec SUVs.