The latest Mercedes S-Class has blown away its luxury rivals, but despite taking giant steps forward in terms of interior quality and technology, fuel efficiency hasn't been quite so remarkable.
That's all about to change, though, with the arrival of this new S300 Bluetec Hybrid model. It uses the same combination of diesel-electric power as the E300 Hybrid – which pairs a 2.1-litre diesel engine with a small electric motor built into the seven-speed automatic gearbox – for a combined output of 229bhp.
Although the battery pack in the boot adds an additional 60kg (pushing the overall weight to more than two tonnes), the S300 emits just 120g/km and returns a claimed average economy of 61.4mpg, making it by far the most efficient four-door limousine on the market.
Currently it's available only in AMG Line trim and with an extended wheelbase, which means prices start at £72,260 – £1,575 more than the equivalent S350 Bluetec V6 diesel. However, a short wheelbase model is due to arrive by the end of the year.
What is the 2014 S300 BlueTec Hybrid like to drive?
Like many hybrids, the new S300 has something of a split personality. At low speeds and around town the 27bhp electric motor does the work, while the four-cylinder diesel unit remains firmly off. Cruising almost silently along like this is really satisfying, and fits in nicely with the rest of the S-Class's refined character.
Refinement is good at motorway speeds, too, but the 2.1-litre diesel engine is not particularly hushed, and the undignified clatter it makes during start-up and under hard acceleration spoils the otherwise relaxing driving experience.
With the diesel engine fired up, the performance is close to that of the S350 diesel model. With plenty of torque available from low revs, this new diesel-hybrid engine does an excellent job of powering the S-Class along at a decent pace, building speed quickly and smoothly.
The sprint from 0-62mph takes just 7.6 seconds - 0.8 seconds slower than the standard diesel model - and the switch from electric to hybrid power is achieved in an impressively quick and near-seamless fashion.
For a car this big and heavy, handling is also impressive. The S300 rides with all the suppleness of any S-Class, with the standard adaptive air suspension cushioning you from all but the biggest bumps in the road.
The steering is light, but gets gradually heavier when cornering at higher speeds, and it's also precise enough to give the driver enough confidence to place this big car accurately on the road.
Apart from the gruff engine, other intrusive noises are all kept to an absolute minimum, and road and wind noise are barely audible in the cabin, even at quite high speeds.
What is the 2014 S300 BlueTec Hybrid like inside?
Utterly peerless. The S300 introduces several new features to the luxurious cabin, with the smartphone-style 'touchpad' from the C-Class fitted to the centre console. It allows you to pinch to zoom, and swipe left and right through the various menus of the COMAND infotainment system. It's not as intuitive as we were expecting - it's still easier to use the rotary dial to control the system - but it will now be fitted as standard to every version of the S-Class.
The S300 hybrid also has an energy flow meter to measure how the power is being shuffled between its two power sources. The display shows how much charge there is left in the batteries, which provides a visual cue to help maintain pure electric driving without the diesel engine cutting in suddenly.
The AMG Line trim comes with an extensive standard kit list, which includes three-zone climate control, 3D sat-nav, a 12.3-inch infotainment screen, hard disc drive, DAB radio, Bluetooth, and full LED headlights.
This extended wheelbase version has a vast amount of room for any rear passengers to stretch out in, with two individual lounge style seats. The car we tested was fitted with the optional rear seat comfort pack (£3,500) which includes a massage function and high-end Burmester hi-fi system.
Up front, the mix of tactile, high quality materials with a vast array of technology makes the S-Class feel years ahead of its closest rivals. All the switchgear feels robust and substantial, and visibility is good, although the thick pillars do create the occasional blindspot when pulling out of tight junctions.
Should I buy one?
Despite having similar theoretical running costs to a petrol supermini, the S300 Bluetec hybrid is still probably the least convincing car in the S-Class line-up. The clattery four-cylinder diesel engine robs it of the whisper-like refinement of the V6 and V8 models, and spoils the overall driving experience.
Rivals such as the Audi A8 2.0 TFSI Hybrid and BMW ActiveHybrid 7 don't come close to matching the fuel economy figures of the Mercedes, but are both more refined and quicker.
The Mercedes S350 diesel might cost more to run, but it's cheaper to buy, quicker and more refined than this new hybrid – qualities that are likely to be highly valued by anyone considering an S-Class.
Buyers looking for the ultimate S-Class hybrid should hold out for the S500 plug-in hybrid due this summer. It will be more expensive than the S300 but will use a V8 petrol engine and have a much longer pure-electric range than the S300. It will also emit just 69g/km and return a claimed 94.2mpg.
What Car? says..
Mercedes S300 Bluetec Hybrid AMG Line
Engine size 2.1-litre diesel, plus electric motor
Price from £72,260
Power 229bhp (combined)
Torque 369lb ft
0-62mph 7.6 seconds
Top speed 149mph
Fuel economy 61.4mpg