Recent events seem to prove that out. Just when German manufacturers have begun to offer clean diesels that meet emission standards in all 50 states, the price of gasoline has plummeted and widened the value gap with diesel fuel. You’ll be burning fewer gallons in your diesel-powered car, but you’ll probably be paying more for them. So you can’t recoup the extra cost of the heavy-duty diesel engine with improved fuel economy.

With those thoughts in the back of my mind, I tested two diesels on back-to-back weekends.

The specifications for the 2009 Jetta TDI got me excited. Here was a German sport sedan (albeit one built in Mexico) with a sophisticated turbo-diesel engine, and bearing a sticker price under $25,000 ($24,190 to be precise).

As promised, the Jetta’s fuel economy is impressive. EPA figures a combined average of 33 miles per gallon, while some drivers say they can approach 50 mpg. There is no free lunch, however, and the Jetta TDI exhibited some flaws. I heard the distinctive “diesel clatter” when the car idled – not a disruptive sound but not desirable either.

More disturbing was the pronounced turbo lag; the better part of a second was required before depressing the accelerator resulted in a noticeable surge forward. It wasn’t the smooth pickup I was hoping for.

Neither of those problems was apparent in my second diesel, Mercedes’ 2009 ML320 BlueTEC. The engine was as quiet as any car I’ve driven, and its performance was as seamless. A well-sorted Mercedes exudes a confidence and sophistication like no other, and this body style has been in production long enough to have its rough edges buffed out.

Once again though, I came up empty-handed trying to find a clear rationale for choosing a diesel over a gasoline engine. The city/highway mileage of the diesel ML is rated at only 18/24, which is not a huge improvement over the gasoline-powered ML’s 15/20.

And I still haven’t adjusted to the aggressive pricing of German luxury cars. While the base price on the ML was tabbed at $48,600, entertainment and heating packages – along with some other geegaws – rolled it up to $58,885. It is hard to feel green with that much precious metal under your foot.

Diesels continue to be a huge deal in Europe, where taxes keep the price of diesel fuel closer to that of gasoline. And they will get a lift in the U.S. next year with the introduction of the Honda Accord diesel. Hopefully, that will be enough to move them from the future back to the present because, based on my recent experience, they aren’t there yet.