Audi TT TDI Roadster

Even after five years on sale, the TT can still compete with the best roadsters. Some buyers will prefer the extra grip of the all-wheel-drive, too. Yet it’s a capable car rather than a thrilling one, while a harsh ride and relatively poor fuel economy also disappoint. The fact the TT TDI is outgunned by the gutsier SLK here seals victory for the Mercedes.

The Audi TT is the original diesel roadster and one of the most recognisable sports cars on the road, so its appeal remains as strong as ever. But will its age hold it back against the powerful new diesel Mercedes?

It certainly looks less showy than its rival from the outside, with a tried-and-tested design that’s as restrained as it is stylish. This could be because our mid-spec Sport version rides on relatively small 17-inch alloys and the front and rear bumpers look plain when compared to aggressive S line models.

Short overhangs emphasise the compact proportions, while the rising shoulder line running from the grille to the rear arches is as distinctive as ever. However, cracks are starting to appear in the TT’s carefully crafted image, as the tall chrome roll hoops and old-fashioned radio antenna betray its advancing years.

It’s a similar story inside. While there’s nothing wrong with the metal bezels for the climate controls or the quality of the materials, the narrow dash is covered in buttons and the £1,685 optional DVD-based sat-nav appears dated compared with the SLK’s COMAND Online infotainment system.

The Audi feels like a sportier car than the Mercedes, though, with figure-hugging bucket seats (a huge £2,530 extra), a flat-bottomed steering wheel and an excellent low driving position. Putting up the fabric hood takes just 15 seconds and, although the black headlining makes it feel dark inside, rear visibility with the roof up is on a par with the SLK’s.

Other practical touches include a decent-sized glovebox – which has a separate shelf for holding your iPod – and a handy storage cubby that’s hidden in between the two front seats. Yet despite these clever touches, the TT isn’t as versatile as the SLK inside – The shallow 250-litre boot is 85 litres smaller than the Mercedes’ with the roof up.

Over our test route we also found that the MMI connection cable kept cutting out over bumps in the road. You might assume that this was just a quirk of our test car, but it’s caused by the TT’s fundamental dynamic flaw – the harsh ride.

Even on small 17-inch wheels and without the stiffer sports suspension, the Audi thumps hard over anything but the smoothest tarmac. This will make the magnetic dampers a must-have option for the vast majority of TT Roadster buyers, despite their high £1,175 price.

The payoff for this uncompromising set-up is that the Audi responds quickly to driver inputs, while the quattro transmission delivers unbreakable traction. However, the 2.0-litre TDI engine is noisy at speed and lacks the in-gear grunt of the Mercedes.

It was less efficient than the SLK, too, returning 37.7mpg in our hands. We’d expect a bit more from a diesel roadster to make it worth the hefty price premium over TFSI petrol versions. It will also cost more in tax than its efficient rival.

Still, the manual TT is a lot cheaper than the SLK to start with – and although some of its sheen has come off with age, it remains a very desirable car. But has it done enough to scrape through for a win?