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Engine options, speed, acceleration and refinement

Although it sounds like a utilitarian throwback to find the Mercedes' 2.1-litre, four-cylinder engine in a car like this, it shouldn’t, given the capability of the ML 250

Its 93bhp per litre isn’t uncommon by modern diesel standards, but it still takes some getting used to for those of us who remember (and it wasn’t so long ago) when 100bhp per litre was considered at the sharp end for naturally aspirated petrol cars. 

A urea solution called AdBlue is injected into the exhaust gas to cut NOx emissions by up to 80 per cent

But the 201bhp peak isn’t only what makes this 2.1-litre engine sufficient in the 2310kg (as tested) ML; it’s also the exceptional 369lb ft of torque, a broad spread of it, and the ability of the seven-speed auto ’box to make the most of the power and torque delivery.

It still leaves the ML 250 looking a touch malnourished compared with some of its bigger-engined rivals, but it would be churlish to suggest that an 8.8sec 0-60mph dash in a car like this is insufficient. 

In-gear flexibility is decent if you hold on to the gearbox’s ratios via its steering wheel paddles; 50-70mph in seventh, for example, takes a respectable time of 11.0sec. But even with the gearbox thus selected, you can kick past holding on to a ratio if you’re determined to give the carpet a hard time. Flat out through the gears, the same 50-70mph benchmark can be achieved in a sprightly 5.8sec.

The real benefit, though, is felt at the fuel pump . In our experience, no other SUV of this size and performance will deliver overall economy into the mid-30s.

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So it’s quick enough and enviably economical. Why would you pick a bigger-engined M-Class?

Because all things being equal, a four-pot will not be as smooth as a six-cylinder unit from the same manufacturer. The ML 250’s noise levels are more than acceptable, but an unmistakable muted grumble can be heard; there’s no denying that the six-pot ML 350 offers a more hushed environment.

That model shaves around 1.5sec off the 0-62mph time and adds 9mph to the top end, meaning flat out the big-engined ML will reach 139mph. Our European drive of the ML350 indicated the demonstrable benefits of six cylinders; the work performed by Mercedes in reducing NVH and increasing refinement is brought to the fore with a smoother powerplant.At the opposite end of the ML range and performance spectrum, the ML 63 AMG offers fearsome performance from its bi-turbo 5.5-litre V8. Its 518bhp fires the 2345kg ML from 0-60mph in less than five seconds. Moreover, its prodigious torque reserves (516lb ft) are available through such a broad rev range (1750-5500rpm) that overtaking is a very easy task.