It feels like I’ve become trapped in a parallel universe where life plays out in mirror image. Earlier this month I left a shirtsleeve-warm London and flew to a Phoenix, Arizona, so cold that journalists attended an outdoor press conference wrapped in blankets.

Then I got up and climbed aboard a BMW with a folding metal roof that more than one BMW engineer had told me the company would not make. And then, despite its twin turbo engine and more than 300bhp output, I didn’t much enjoy driving it.

So if I return home and discover friendly staff at Heathrow and no roadworks on the M4, I doubt I’ll be surprised.

It’s not often a car manufacturer goes out of its way to lower expectations of a car before you drive it, but BMW did a good job with this new 3-series convertible.

Albin Dirndorfer, its project manager, claimed convertible drivers were more interested in a “relaxed” drive than a sporting one, and then went on to point out that the coupé on which it is based met “much higher dynamic requirements than the convertible”.

I was rather heartened by this. Indeed I applaud BMW for daring to say what other convertible manufacturers know but still absurdly try to cover up: namely that there is not now, nor has there ever been, a convertible that’s as good to drive as the closed car on which it’s based.

The reasons are always the same: convertibles are structurally less rigid than coupés and, in an attempt to ameliorate some of the worst symptoms of this lack of torsional rigidity, manufacturers reinforce the chassis, making the cars much heavier. In fact the convertible 3-series weighs 463lb more than the coupé.

To understand what effect that might have on its performance, handling and economy, imagine how your car might behave with two baby elephants on board. Exactly. In fact once you’ve added an average driver, the convertible is heavier than an empty 7-series limousine.

What’s remarkable, then, is how little this appears to have dented its performance: the top-of-the-range 335i model is just 0.3sec slower to 62mph, its top speed is the same electronically limited 155mph and its fuel consumption has fallen just 1.2mpg. This, however, is not how it appears on the road.

Despite the fact that convertibles usually feel quicker than they are thanks to their proximity to the elements, this 335i never felt like it had 306bhp under the bonnet. When I drove its coupé sister last year I described its performance as relentless. This one never graduated beyond pleasantly quick, and you can thank the elephants for that.

BMW reckons this lack of knuckle-whitening excitement will trouble its customers little, not least because it’s quicker than its immediate rivals, and the company is probably right. Try as I might I cannot see a prospective 335i convertible buyer crying into his or her chardonnay about the adverse effect of the roof on its power-to-weight ratio. It’s more likely they’ll complain that it looks a trifle dull with the roof down and more than a trifle dumpy with it raised.

As Volvo did with the C70, BMW has taken the more difficult but theoretically more aesthetically pleasing option of a roof with three short sections instead of two long ones, but the Volvo is, to me at least, substantially the prettier of the two. And that’s without mentioning Audi’s visually gorgeous soft-top A4 Cabriolet, which is better looking by far than either of its hard-topped opponents.

Still, I hadn’t been at large in Arizona for long before my initial disappointment ebbed away. Roads that stretch arrow-straight for miles into the desert come with 40mph limits and a bewildering number of state troopers with the will and the way to enforce them. One was kind enough to show me his new Taser and proudly announced that not only would it put 50,000 volts through a man 21 yards away, but that it had done just that only yesterday; suddenly I wanted to be extra law abiding.

Instead of putting my foot down I sat back and cruised and, while I don’t associate small 300bhp BMWs with this activity, it is by far the strongest suit of this particular one. It rides beautifully, shakes little even on rough surfaces and manages the airflow around the cabin exceptionally well.

I’d brought no warm-weather gear because who would in the Arizona desert? Yet driving with the roof down in temperatures barely above freezing was no hardship at all. It steered nicely into the few corners there were and took me round the state with a minimum of fuss. Which was all very pleasant.

But while I may seem to damn with such faint praise, the truth is BMW has done as well as can be expected. The Audi S4 Cabriolet is heavier and slower than the 335i convertible, despite having a fabric roof, and while I haven’t driven them back to back I have no doubt which I’d prefer. Likewise the Volvo C70, even in flagship T5 guise, is about as exciting as a tax form.

Remember, too, that this £37,895 335i SE version is as exciting as the 3-series convertible is likely to get. It goes on sale on March 24 with the 218bhp £33,030 325i, while the 320i, 330i and diesel 330d needed to complete the range arrive in the summer.

Dirndorfer, who was also responsible for the 3-series coupé, finished his presentation by explaining why he cannot answer the question he gets asked the most: which does he prefer, the coupé or the convertible? “It is like being asked to choose between a sporty son or a beautiful daughter,” he claimed. Leaving aside the daughter’s undoubted beauty, it’s the son every time.

The convertible may be good by chop-top standards — and it’s probably the best in its class — but the coupé feels like a proper sporting BMW, something this convertible may aspire to but never comes close to achieving.

VITAL STATISTICS

Model BMW 335i SE Convertible
Engine type 2979cc, six cylinders
Power/Torque 306bhp @ 5800rpm / 295 lb ft @ 1300rpm
Transmission Six-speed manual
Fuel/CO2 28.5mpg (combined cycle) / 238g/km
Performance 0-62mph: 5.8sec / Top speed: 155mph
Price £37,895
Rating 4/5
Verdict Best in class, which sadly isn’t saying much
Date of release March 24

THE OPPOSITION

Model Audi S4 Cabriolet £43,025
For Great looking, well-built, quick
Against Little fun to drive, expensive

Model Volvo C70 T5 £33,250
For Great looking, smooth engine
Against Limited handling, performance

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