Are you thinking of ordering a new car? Are you about to pick another diesel, assuming you’ll save money by choosing the car with the best fuel economy? Stop. Get the calculator out. Examine the figures for the latest breed of super-efficient petrol cars. Do the maths, and prepare to be surprised.


Sales of diesel cars have soared in recent years, and with good reason. They have become extraordinarily refined and even more efficient. They can’t match powerful petrols for top-end power but the best diesels offer huge low-end torque that delivers an instant, addictive surge of acceleration from low speeds. It’s often more than a match for some big V8 petrol engines, and is more relevant and usable than top-end power on congested, speed-controlled roads. Plenty of drivers now pick a diesel because they simply prefer the way it drives. Across Europe, diesel accounts for around 53% of all new car sales but it dominates certain market sectors; the bigger hatchbacks, saloons, estates and SUVs are overwhelmingly diesel-powered. 


But diesel’s growth has slowed in the past couple of years, and some predict it will soon be thrown into reverse. Diesel cars are typically more expensive than the petrol alternative, by an average of around €1,600, much more in the case of luxury cars. That price differential is likely to grow, as hitting new EU emissions targets will add more to the cost of building diesels than petrols. 


At the same time, a new breed of petrol engines is closing on diesel’s economy advantage. How quickly you recoup the extra initial outlay on a diesel depends on the relative cost of the two fuels where you live, and how much more efficient the diesel is. As cars like the Mercedes-Benz C180K BlueEfficiency narrow that margin, the economic argument for a diesel gets harder to make. In some cases, it’s now impossible.


So why are petrol engines getting so much cheaper to run? The secret lies in using a smaller capacity engine, but with a supercharger or turbocharger (or both) to produce the same performance but with lower emissions and fuel consumption. The C180K is the perfect example. A 1.6-litre petrol engine doesn’t sound like it’s going to produce the kind of performance you’d expect in a premium small saloon or estate. But it matches the power and torque of the previous entry-level petrol engine, despite being 200cc smaller, and cuts emissions by 19% and fuel consumption by 11%. Mercedes’ BlueEfficiency branding is applied to models that have been eco-tweaked to improve economy. In addition to the improved engine, the C180K gets low rolling-resistance tyres, a slightly more aerodynamic exterior, and a shift indicator that helps you stay in the best gear for fuel consumption. Many of the mainstream carmakers have these eco sub-brands, but few yet apply them to petrol engines.


Before we look at how the C180K drives, let’s look at how the numbers stack up. We’ll be kind to diesel, and compare it with Mercedes’ entry-level C200CDi diesel option, which costs around €1,200 more than the petrol depending on market, and has fuel consumption of 5.7l/100km to the petrol’s 6.3. In reality, most buyers would be choosing between the petrol and the next diesel up, the C220CDi, which costs around €2,400 more and is a little less economical at 5.9l/100km, but is closer on performance.


In those markets where diesel is more expensive, such as the UK, Greece, Sweden and much of eastern Europe, the C200CDi’s efficiency advantage is almost wiped out. Its cost-per-km advantage over petrol is just a fraction of a cent, making it effectively impossible to recoup the extra purchase cost of the car. In the UK, you’d need to be driving an interstellar 190,000km each year to break even over three years of ownership. In Germany, where petrol is more expensive by around €0.07 per litre, you’ll still need to be covering more than 38,000km each year for the diesel to make sense. Even in the Netherlands, where petrol is more expensive by the greatest margin at around €1.30 per litre to €1.03 for diesel, you’ll need to be covering nearly 18,000km each year, or around 2,000km more than average, just to break even if you pick the diesel.


But while a downsized petrol engine might seem appealing in a supermini or small hatchback, could you really live with one in a Mercedes? Frankly, yes. The C180K’s 156PS power output isn’t going to set your trousers alight but its 9.5-second 0–100kph time and 223kph maximum speed are more than sufficient, and in daily use it never feels short of power; the engine is torquey and flexible and has an engaging but subtle supercharged whine as you extend it to the redline. And no matter how refined diesels get, they’ll never entirely eliminate their loud chatter when idling, particularly when cold. A good petrol will always feel more refined.


You can maximise your fuel efficiency by choosing the C180K with the manual gearbox and the SE or Elegance trim, rather than the Sport with its firmer suspension and wider, less eco-friendly tyres. Both choices bring other advantages: as well as being cheaper the manual gearbox offers a slick, short shift, and the more comfort-biased suspension offers a remarkably pillowy ride over poor surfaces. Overall, the C-class range is exceptional; it’s a match for the rival BMW 3-series for dynamics and the Audi A4 on build quality. The styling is remarkably sexy and sharky for a baby Merc, and if you’re not quite ready to downsize to a 1.6, the engine range goes all the way up to a monstrous 457PS, 6.3-litre V8.


But the C180K is by far the more relevant, significant engine. Other carmakers are working on similar technology; Volkswagen combines smaller capacity with both supercharger and turbocharger on its award-winning TSI engines, and Fiat will soon introduce its radical, turbocharged two-cylinder MultiAir design. We’re not predicting the death of diesel anytime soon, but petrol is making rapid advances. Do the maths, and see if it makes sense for you.

Mercedes-Benz C180K BlueEfficiency



Price: from £22,858 (UK) 


Engine: 1.6-litre 4-cylinder


Power: 156PS


Top speed: 223km/h


0—100km/h: 9.5 seconds


CO2: 149g/km


Economy: 6.3l/100km


We like: Economy, emissions, costs


We don’t like: Lack of bar-room 
boasting potential


Verdict: Makes a premium-badged, petrol-engined car socially 
acceptable — and cheap to run