The new Mercedes S500 Plug-in hybrid is the most technologically-advanced version of the word’s most hi-tech car, a spacious and super-luxurious saloon that’s as fast as a Porsche Boxster S from 0-62mph, yet returns more than 100mpg and emits just 65g/km of CO2.

The technology under the skin is far from simple, but the principle is straightforward. By fitting the S500 Plug-in with a battery ten times the size of the its more conventional siblings – the S400 Hybrid and S300 BlueTEC Hybrid - it can run on battery-power-only more of the time, and therefore drink less fuel. Plug it into a normal UK household socket and a full charge takes a little under three hours, giving a zero-emissions range of up to 20 miles.

The powertrain is a development of Mercedes existing series-hybrid technology and combines a 328bhp 3.0-litre twin-turbo V6 engine, with a 114bhp electric motor fully integrated into the seven-speed automatic transmission. As a result, select the ‘Sport’ driving mode and there’s V8 levels of shove whenever you need it, delivered in one smooth wave of torque. A 0-62mph time of 5.2 seconds is only 0.4 seconds slower than the S500, despite weighing 200kg more.

It’s in Eco and Eco+ driving modes though where the plug-in really comes into its own. The car’s computer is constantly juggling the two power sources for maximum efficiency, only calling on the V6 when you bury your right foot or run out of charge. In Eco+ the ‘haptic accelerator’ will vibrate under your foot twice when it senses a slower car or object in front, advising you to come off the throttle and coast before you have to hit the brakes.

Pushing the brake pedal first reverses the electric motor for deceleration and then activates the traditional brakes when you push harder. Information from the sat-nav on the road gradient is also fed in to the computer so the car can deplete the battery on uphill stretches, knowing it can be recharged on the downhill section.

Plus, the GPS can fire up the engine while you’re heading towards a city centre, so the batteries are fully charged when you get there.

There’s also a moveable resistance point on the accelerator in electric-only E-Mode – go past it and the engine fires up, removing the guess work usually associated with careful zero-emissions driving.

As well as E-Mode, there are three other operating modes – Hybrid, E-Save and Charge. This allows you to manually override the computer and decide whether you want to maintain your charge level with E-Save (useful if you want to preserve a top up from the mains to use in town later on) or in Charge mode you can increase the load on the engine to refill the batteries for an extended E-Mode drive later on.

With 240kg more to carry around than the equivalent S350 BlueTEC diesel (the S500 Plug-in is only available in long wheelbase and AMG Line trim) there are compromises on the road. In E-Mode it’s not exactly fast (although there’s enough performance for wafting silently around town) but only a Rolls-Royce Phantom can match this car for comfort, quietness and refinement – there’s no more soothing way to travel. Call on the engine, for an overtaking manoeuvre for example, and it fires up quietly and revs smoothly, although it never feels as fast as the figures suggest.

You can sense the extra weight in the corners, too, but if you want burnouts and oversteer then the S63 AMG will be more up your street. The Plug-in is about taking things smoothly and serenely and seeing how little fuel its possible to burn.

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