The Mercedes-Benz S-Class may not be as sporty as rivals like the BMW 7-Series, Porsche Panamera or Audi A8, but the S-Class is generally revered as the flagship sedan that sets the standard for luxury among its peers. Over the summer, I test drove the Mercedes-Benz S400 Hybrid. While I enjoyed that car’s luxurious interior, I couldn’t help but think that the hybrid S-Class lacked the green factor that usually attracts shoppers to hybrid cars.

Granted, the S400 Hybrid is the lease expensive S-Class you can buy, but gas-only competitors like the A8 and Panamera get similar ratings from the EPA and carry a significantly lower base price. This made me wonder if S400 shoppers buy it because it’s the cheapest S-Class, or because they think hybrid badges are enough to please Mother Nature.

Well, if I was skeptical before, Mercedes has just made me even more suspicious. The automaker just introduced the diesel-powered S350 Bluetec, which I recently took on a short drive around Washington, D.C. With a base price of $92,550, the S350 Bluetec costs just $700 more than the S400, but it gets better fuel economy and comes with standard all-wheel drive.

Fuel economy estimates for the S350 are 21/31 mpg city/highway, which is significantly better than the S-Class Hybrid’s 19/25 mpg city/highway. The EPA estimates that you’ll save $358 per year on fuel with the S350, but if you’ve got the cash to consider an S-Class, fuel costs likely aren’t your primary concern. However, convenience might be a factor, and you’ll travel 86 miles farther on each tank of fuel if you go diesel instead of hybrid.

Then there’s the fact that all-wheel drive is standard on the S350, which should make it more appealing to shoppers who live in snowy areas. All-wheel drive isn’t available at all on the S400, and although it’s optional on the pricier (and less fuel-efficient) S550, you’ll have to pay an extra $3,000 to get it.

With all that the S350 has going for it, I can’t see a reason to recommend the S400 Hybrid. For the moment, the S350 Bluetec and the Porsche Panamera S Hybrid stand as the most fuel-efficient super luxury cars, but who knows what competition lies down the road. Maybe BMW is working on a plug-in hybrid-biodiesel version of the 7-Series. Now that would be cooking with gas.