I like diesel. In certain circles such an admission still has people smirking quicker than owning up to an admiration for Barclay James Harvest. But such detractors will soon be laughing on the other side of their faces.

A diesel uses less fuel than a petrol car, drastically cuts company car tax on bigger cars and for private buyers will always hold its value better than a petrol-powered equivalent. Diesels offer more usable performance, their mid-range torque proving more effective in the real world than petrol cars’ top-end power. For almost all reasons and for almost all purposes diesel has the potential to be better than petrol. Almost.

Allow me to introduce the Audi A4 Cabriolet 2.5 TDI, the first and only diesel convertible on sale.

I like the A4 Cabriolet: I respect its engineering integrity, admire its brilliant cabin and quietly covet it for its looks. And you know already how I feel about diesel. My problem is with their marriage.

In fact, I started having problems with it before I’d even sat in the car. I heard a rattling outside the sitting room one morning and knew without looking that it had arrived. I went to the door, saw a beautiful convertible and could not equate that sight with the sound that assailed my ears.

On paper, the arguments are almost all in its favour. At £26,385 the diesel A4 convertible is a little (£1,715) more expensive than the petrol 2.4 litre version. While both cars are powered by V6s the diesel 2.5 has 163bhp, just seven fewer than the petrol, but it also has 229lb ft of torque, some 59lb ft more than the 2.4. Audi claims the diesel is just 0.2sec slower to 62mph (9.9sec) and has a top speed of 140mph, one more than the petrol car. In reality, however, all that torque ensures the diesel is much, much quicker.

The financial case is unarguable, too: compare combined fuel consumption of 39.2mpg for the diesel with 29.1mpg for the petrol or a company car tax bill that saves a higher-rate taxpayer £334 per year despite the higher purchase price.

Unfortunately, these are sensible arguments for a car that is unlikely to be bought for sensible reasons. People who buy cars like this want you to think they’re free spirits who love feeling the wind in their hair. In fact, all they really want is to show off and they may feel their carefully crafted image can do without a loud rattle to announce their arrival.

Oddly enough, I don’t think diesels need to be a complete turn-off and should BMW ever decide to install its sublime 3 litre six in the 3-series convertible (as it shortly will in the coupé), I would expect it to fare much better. Not only is the engine more powerful, economical and tax efficient than this Audi alternative, critically for such cars it is massively smoother. Waft up to the pub or someone’s house in a BMW diesel and they’d probably not guess which pump it was filled from. In the Audi you’d never get away with it.

The car is easy to fault in other areas, too: it’s 187lb heavier than the petrol 2.4 and it’s all under the bonnet, making the car even more nose-heavy. By BMW convertible standards it feels ponderous when driven fast yet it rides less well too. And while the diesel rattle subsides once you’re up to speed, the engine never threatens to offer more than modest performance, at least not with the otherwise superb optional CVT (continuously variable transmission) fitted to the test car.

Clearly, the union of car and engine was well meant but the road to hell is paved with good intentions. Though the A4 soft top is a fine example of its breed and the engine is perfectly capable, they were not meant for each other. Would you wear a Paul Smith suit with Timberland boots?

Vital statistics

Model Audi A4 Cabriolet 2.5 TDI
Engine type V6, 2496cc, turbocharged
Power 163bhp @ 4000rpm
Torque 229lb ft @ 1400rpm
Transmission Five-speed manual or Multitronic CVT
Suspension (front) Independent, four links, coil springs, anti-roll bar; (rear) Independent multi-link, coil springs
Tyres 215/55 R16
Fuel/CO2 39.2mpg (combined), 197g/km
Co car tax £2,412 for a higher-rate taxpayer
Top speed 105mph
Acceleration 0 to 62mph: 9.9sec
Price £26,385
Verdict World's first diesel cabrio is brave but unconvincing

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