What\'s the last thing you expect to find on the dashboard of a car produced by Mercedes\' bad boy tuner AMG? A shiny little green-lit ‘Eco\' button, that\'s what. Yet there it is, and it\'s serious. Much emphasis was placed on the fact that the engine in the front of this new 2012 SLK 55 is the most efficient such unit currently on sale anywhere. AMG is deeply, proudly committed to this, relentlessly pursuing efficiency across its range in order to reduce fuel consumption and CO2 emissions by a targeted 30 per cent without sacrificing an iota of performance. Engineers say the new SLK 55 AMG will deliver fuel consumption of just 8.4 litres-per-100km, reaching the target. Impressive. But the question I really want to ask is, \"How well does it go, mister?\" Which is why I\'m sitting in the car, in the pit lane at Yas, crash helmet on head, waiting for lead driver Sean to take us out on track. And he\'s no slouch, currently sitting at No. 2 in the Radical SR3 Championship, so this should be fun. Regular readers will know that wheels really rates this latest generation of the SLK, though it has taken a long time for it to shed the girly image and evolve into a serious contender. But evolve it has, and contenders don\'t get much more serious than one with 5.5 litres of AMG\'s finest stuffed under the bonnet. It\'s a recipe that particularly appeals to me — take a small, strictly two-seater roadster, and shove an unfeasibly large engine in the front. Gotta be good for a laugh. In fact, despite the familiar number this is a completely new 5.5-litre V8 engine, developed uniquely for the SLK. It is at heart the same 5.5-litre unit found in the monster 63s, though there it is bi-turbocharged. Here it is normally aspirated, which promises much for response, and in this installation turns out a very healthy 422bhp and 540Nm of torque. The clever bit? Under light load the V8 engine can become a V4 as it deactivates half of its cylinders, though in reality, you\'ll be hard pushed to tell when this happens. Certainly, the first impression is just how alive and responsive this thing feels. We\'re only trundling through the pit lane exit tunnel, but already the car feels as eager as I am to get out onto the track for real and start exploring the envelope. We\'re fortunate tonight to be driving almost the full Grand Prix layout, including the technical ‘street\' section around the marina and through the hotel. Only the top hairpin is excluded, so we go down through the corkscrew instead and out onto the long main straight at about one third of the way along. Plenty of opportunity for the SLK 55 AMG to stretch its legs. And stretch them we do. Response from the non-turbo engine is instantaneous; a real thump in the back as the car flings itself at the horizon. This combination of big engine and small chassis is very seductive. I have no idea what the top speed of the SLK 55 AMG is, but saw some impressively large numbers on the way down to that bottom corner. The shove is just relentless. Fortunately, the brakes are equally impressive. You can really stand on them and scrub off a lot of speed very quickly. Stopping from such high speeds is a little squirrelly due to the short wheelbase, but there\'s no question about the effectiveness of the set-up. Our car was on standard steel discs, which seemed completely unfazed by the punishment they were getting. No fade whatsoever, despite hauling down from warp speeds turn after turn. Handling is pin sharp, with virtually instantaneous turn in and response. The SLK 55 AMG is the lightest of the Mercedes high-performance range and feels neat and nimble, almost go-kart like in its responsiveness. New to the SLK 55 AMG is a brake torque vectoring system that helps with cornering performance by selective brake actuation at the rear wheel on the inside of the bend. For ultimate performance there is an optional Performance Pack, which came fitted to our car. This adds stiffer suspension, an AMG rear axle differential lock, two-piece brake discs at the front and a three-spoke steering wheel finished in leather and Alcantara. Certainly, it is a confidence-inspiring place to sit. The SLK 55 AMG changes direction with alacrity, holds your chosen line well, will oversteer predictably if you overdo things, and the electronics will protect you from 90 per cent of your foolishness. There were strict instructions to keep stability control systems switched on, and you could feel them intervening if you got too exuberant and suffered a surfeit of ego over-ability. That happened to me a lot. In fact, and it shouldn\'t come as a surprise, the easiest way to go really fast in the SLK is smoothness and flow. It is such a small car the temptation is to come charging up to a bend, stomp on the brakes, monster it round and floor the throttle again. Sounds impressive, looks dreadful. The real secret is to work with the suspension and prodigious grip available, and carry more speed into and through the bend, using the whole width of the track. There\'s a tricky little left/right/left sequence just after the second grandstand, and I think I got it right precisely once during our drive. Smoothness offers you better control, and earlier application of power. Getting that power down on the road is simplicity itself. Drive goes to the rear wheels through what\'s called the AMG Speedshift Plus 7G-Tronic automatic transmission. It is in fact a perfectly conventional torque converter auto, no dual-clutch technology here. It does however have some very sophisticated control software and offers three transmission modes: Controlled Efficiency (C), which enables the cylinder deactivation mode, Sport (S) and Manual (M). This last is interesting. You can change gears using the wheel-mounted paddles in all three modes — in C and S, the gearbox will revert to auto shifting after you have done so. But M is different, and once you have selected a gear, it will hold onto that ratio right up to the redline, and until you select another one. Great for cornering, and it gives the SLK 55 AMG a lovely sporting character for when you really want to play. Away from the track there\'s also a lot to like about the new AMG. It has enough luxury, toys and creature comforts to qualify as a true grand tourer, and should be both comfortable and economical on longer journeys. Leather trim is standard, and you get the full range of electronics including Bluetooth streaming and 10GB of storage space. Starting is keyless and push-button, the stereo sounds great, and the whole interior is squeak and rattle free. In fact, such is the integrity of the shell that even during maximum attack on the track you are not aware that you\'re driving a convertible instead of a closed-roof coupé. Our particular car came with a Teutonically black interior, relieved only by a few silver highlights, and rode on very striking dark grey light-alloy wheels. There was also a smattering of chrome garnishes on the flanks, and chrome tips on the quad exhaust pipes. But there is one particular detail that most people will never see. This is a true AMG, and in the AMG tradition, every engine is hand assembled by a single builder, who then puts his name on it. In our case, take a bow Mr Torsten Oelschläger. You built a beaut. Mercedes took its time to make the SLK into a car that could be taken seriously. AMG has taken it much further, and though it remains its ‘smallest\' car, this baby of the family is the perfect introduction to the brand. At the end of the day, few cars will leave you with a bigger grin on your face than this. Nobody needs a 5.5-litre V8 in a two-seater. That\'s what makes it such good fun. Divine madness.