Indulgence meets rationality in the new Mercedes-Benz C-Class Cabriolet in diesel form. It comes with fine looks, an accomplished drive and palatable economy and emissions; you could even convince yourself it's practical such is the passenger and boot space on offer. The perfect all-rounder in the new Cabriolet line-up, if lacking the outright glamour of its petrol-engined relations.

In the metal

Mini S-Class Cabriolet styling here: Mercedes-Benz's design chief Gorden Wagener says that he wants a clear familial look and he's got it in the new C-Class Cabriolet for sure. It works better in AMG Line trim, bringing a bit more glamour to the design, an AMG styling kit combining with the diamond point grille and a 15mm drop in the suspension. Unlike its key BMW rival the C-Class Cabriolet retains a canvas roof, all the better for showing off even when the sun's not out - especially when optioned in a contrasting colour. That fabric roof has packaging advantages, too, though there's no denying it looks a touch ungainly with the roof in position, so the C-Class is more elegant with its top stowed away.

Do that and you reveal a large cabin to the elements, the C-Class a genuine four-seater, though even with the air diverting foil deployed at the top edge of the windscreen you might find it a bit draughty in the back at speed. That air-shifting kit does nothing for the C-Class's lines, unfortunately. The interior is as we've come to expect from the current Mercedes-Benz line-up, with prominent, circular air vents, a tablet-like screen in the centre console and large circular instruments supplemented by a central screen. It all looks, and crucially, feels, of high quality, the seats providing plenty of comfort and there's even a well-shaped and sized boot - should practicality be on your list of cabriolet buying requirements.

Driving it

If you're after the most engaging and entertaining C-Class Cabriolet you'll be better served elsewhere in the range; specifically with the new AMG C 43 model and its even more unhinged C 63 relations. The C 250 d is a capable all-rounder, mixing useful economy and emissions with agreeable performance. There's plenty of low-rev torque (the 500Nm peak delivered from just 1,600rpm) and with nine gears on offer from the new automatic transmission it can easily keep it in that rich seam of twist for maximum economy and fine performance. There's no real benefit from piling on more revs, or, for that matter, using the paddle-shifters to swap those numerous ratios yourself - the automatic does a fine job itself of doing that.

It's quick enough, but the 7.2 seconds it takes to reach 100km/h doesn't reveal its decent mid-range shove, and this isn't a sprinter, more a long-legged, cruiser with the emphasis on relaxation. Ask for more and you'll increase revs, without a significant corresponding increase in pace, while also adding some additional mechanical chatter from the percussive 2.1-litre diesel engine. Likewise there's little incentive to alter the drive modes; Eco or Comfort are fine, Sport or Sport+ hold onto gears too long and detract from the refinement.

What's immediately apparent is how stiff the C-Class Cabriolet's structure is. Given the large opening it presents to the sun it's very taut, so it takes a particularly severe surface to eke the merest squeak from the roof while up, and the rear-view mirror is commendably free of the shake that betrays some floppiness in the structure of its rivals. That's all to the benefit of sharpness, as the C-Class Cabriolet is agile, surprisingly so, given its not insubstantial bulk - weighing 1,740kg in C 250 d from. The suspension does a fine job of delivering a compliant ride, mixed with sophisticated control, though the steering on the diesel feels fairly inert, lacking the linear clarity and fine weighting that's delivered by its AMG relations.

What you get for your money

Prices for the new C-Class Cabriolet start at €54,580 for the C 180 petrol model with a 1.6-litre engine and a manual gearbox. The C 200 is a better bet at €55,900 with its 2.0-litre petrol engine. Diesel power starts with the noisier C 220 d, at €56,395. Trim levels are standard and AMG Sport.

Alternatives

Audi A5 Cabriolet: long in the tooth now, its replacement is imminent. It still looks great though, and given it's on the way out you might be able to pick up a bargain. The Audi diesel is quieter than the Mercedes rival too.

BMW 4 Series Convertible: uses a folding hardtop and delivers a sharper driving experience. The BMW's four-cylinder diesel also has an edge on the Mercedes-Benz. It's not as spacious though.

Ford Mustang Convertible: not the most obvious rival, and no diesel option, but could appeal to those that want something a little different.

Summary

The diesel is not the C-Class Cabriolet your heart would have you buying, but about as rational an open-topped purchase you could purchase. It drives well, looks good, is spacious and should be generously appointed. The old 2.1-litre turbodiesel isn't the finest engine in its class, however fine a package it's installed in.