Road Test: Mercedes-Benz SLK 250 CDI 2dr Tip Auto

This is it - diesel has officially, in an entirely unofficial Top Gear announcement, conquered the world. Merc's small, rear-wheel-drive roadster was one of the last bastions of the petrol engine, but now even it has fallen to the power of the black pump.

Mercedes has dropped a 2.1-litre diesel into the SLK, so genteel lunch parties up and down the land will now start with a bit more rattle than before. And, yes, we know Audi does a TT Roadster with a 2.0-litre diesel, but the SLK was at the forefront of the premium-roadster revolution 15 years ago, and somehow an SLK with a diesel is a more significant step. And besides, the TT isn't rear-wheel-drive.

Plus, Audi's sales show what a risk Merc is taking - of 1,572 TT Roadster UK sales last year, just 281 were diesel. So the Mercedes step is a curious one. The engine is the four-cylinder, 2.1-litre that appears in every Merc from a C-Class to a CLS, and in SLK tune it produces 201bhp and 369lb ft. More significantly, the SLK CDI does 56.5mpg and emits 132g/km - that's 13mpg and 19g/km better than the next most efficient SLK and is superior to the TT's 53.3mpg and 139g/km, too.

With all that torque, the SLK is a quick car. The seven-speed auto (standard on the Merc) suits the lazier diesel, but if you want to get a shift on, the SLK is capable. Zero to 62mph takes 6.7secs, and, if anything, the in-gear punch (as ever with a diesel) is even more impressive. Torque builds smoothly from 1,400rpm, so you're always in the power band, and once you're rolling at 30mph-plus, there's good, prompt response and power on tap. Ease off, and the engine settles into the background nicely. Even the stop/start function is OK; it fires the engine up smoothly and quickly.

But here's the rub - small premium roadsters, and the SLK in particular, have always been better at pottering than blasting. The diesel's power delivery suits this, but the rattle doesn't. The 250 CDI is simply too rough at idle and at low speeds, which spoils what is otherwise a good car. As far as diesel roadsters go, we'd have the SLK over the TT every time. But until Merc's four-cylinder diesels become more refined, we'd still rather have a petrol SLK, even with the higher running costs. Turns out diesel isn't the answer to everything.