The Silver Arrow has made some S-class-sized waves with its S400 Hybrid. As my colleague Lawrence Ulrich reported yesterday, the car blends the unabashed luxury of Mercedes-Benz’s yacht-sized sedan with smidgen of green penance and unexpectedly good gas fuel efficiency.

Now it looks like the German carmaker is looking to seriously step up its green game with its new S500 Plug-In Hybrid. With just a few tweaks, Mercedes-Benz says it has managed to bump the 4-door’s fuel economy to a Prius-busting 73(ish) mpg, and still provide a 5.5 second zero-to-60-mph time. Now this is a hybrid we can get behind.

Under the hood, Mercedes-Benz has stuck with the same drivetrain as found in the S400 Hybrid right now. That means a gasoline V6 engine provides most of the pep, while a 60 horsepower motor nestled between the 6-cylinder and the transmission joins the fun. The biggest difference between the two lies in the battery department. While the S400 Hybrid gets a 0.9 kilowatt-hour battery, the S500 makes use of a significantly larger 10 kilowatt-hour lithium-ion pack positioned behind the rear seats. The change allows the S500 Plug-In Hybrid to travel up to 18 miles on full electric power. For comparison’s sake, that’s around six miles more than Toyota’s plug-in Prius, and in a much more attractive wrapper.

Once those cells are depleted, though, don't expect to be able to pack up and head out in a hurry: Mercedes-Benz says the batteries can be replenished in around an hour using a special charging station, but if you feel like plugging the S500 Hybrid into a standard wall socket, it will be 4.5 hours before this saloon is ready to hit the road. That seems like a long wait, but when you think about how much time your vehicle spends sitting in your garage or in your parking space at work, getting by on a single tank of fuel for a month doesn't require an unreasonable stretch of the imagination.

So how likely are we to see plug-in Silver Arrows cruising around town? Merc says it will be awhile before the tech hits the street. While the battery pack in the S400 tucks neatly into the engine bay, the considerably larger pack in the S500 requires a good bit more space. The company’s engineers are still scratching their heads trying to figure out how to maintain handling and storage benchmarks (something the company takes pretty seriously, obviously) while making room for the 287 pounds of lithium. Given the looks, the fuel economy and the marque that's marrying the two, we're willing to wait.

(Photo source: Mercedes-Benz)