Road Test: Mercedes-Benz C Class C250 CDI BlueEFF AMG Sport 5dr Auto [Comand]

We have become spoilt. Had, 25 years ago, Mercedes announced a car with more torque than the ‘Hammer' E-Class AMG and in-gear acceleration to flatten a supercar, but which returned 55.4mpg and 134g/km of CO2, we'd have hailed the Germans for mastering in-car nuclear fission or the mystery of kryptonite.

But now? Now, we look at those numbers and think, ‘Yeah, very nice, but any chance you could make it sound a bit less... dieselly?'

So let's ignore the rattle and instead concentrate on all the good stuff in the C250 BlueEfficiency, the latest diesel from Merc's eco-range and perhaps the most convincing blend of fastness and frugalness we've yet seen.

Under the bonnet is the same 2.2-litre, four-pot turbodiesel in the E250 CDI. Even in the big E-Class, it did a more than adequate job - thank sequential turbocharging and 368lb ft of torque (that's more than an F430) - but it turns the C into an absolute flier, a proper performance saloon.

It'll hit 62mph in seven seconds flat, but even that impressive stat doesn't do justice to the in-gear acceleration. Drop into fourth at, say, 40mph and there's a real pin-yer-ears-back thump of torque as the speedo zips up to UK-illegal speeds at a frankly unseemly pace.

Yes, there are plenty of other quick diesels out there - Mercedes's own C320 CDI is a rapid thing, for one - but it's the combination of performance and economy that puts the C250 right to the head of the eco-pack. Even the BMW 325d can't get close, taking a few tenths longer to hit 62mph while emitting over 150g/km of CO2 and returning 49mpg.

The C250 isn't perfect. Our test car was fitted with 17-inch alloys and the optional, lower sports suspension, which combined to produce a disarmingly gritty ride quite out of character with the C's motorway-devouring nature. Smaller wheels and the standard suspension set-up should sort that, but worse is the manual gearbox, which feels brittle and notchy, a flaw exacerbated by the high-biting clutch. The auto gearbox will suit the C250 better, but sadly that adds around 20g/km of CO2 - and a vital tax band.

Then there's the noise, which, even by diesel standards, is as agricultural as Norfolk: not for the Mercedes C250 the mellow thrum of BMW's 6cyl diesels or the hushedness of the French four-pots. But spec smaller wheels, splash those few quid you've saved on a box set of ‘The Great Noisy Supercar Engines Ever! Volume 14" on CD, and here's proof that you can indeed have your environmentally friendly cake and eat it. Albeit noisily.