It’s my job, each week, to come here and write about flowers, frogs, foxes and fornication and then, towards the end, say a little bit about the car I’ve been driving. It is not my job to tell the motor manufacturers what to do.

Some of my colleagues in this auto journalism malarkey are an extension of the car industry, shaping its policy and directing future operations. They are clever. They can understand and explain torque. I can’t. I’m just a punter, test-driving cars and saying whether I like them or not.

Normally, then, I would say that the satellite navigation system used by BMW is rubbish and move on. But with petrol at £400 a litre, we can’t afford to be wasting the stuff by driving to the shops via Dorset every morning. So, today, I shall break with tradition and urge BMW to talk to its sat nav suppliers, with some steely-eyed, Germanic sternness.

The system in the M3 I had last week did not know about the A43. It has no clue that the M25 is connected to central London by the A40. And it had never heard of the Fosse Way, even though it’s been around for 2,000 years.

Last Wednesday, I needed to drive from London to Corby, which, in my mind, was just a few miles from the A1. But the madman in the M3’s dashboard had never heard of the Great North Road and was adamant I should use the M1. So I did.

Big mistake. Back in 1959, the M1 was a wondrous thing, a big grey superhighway for people on the move. It had a point. It had a purpose. Back then the government took our money in taxes and used it to make our lives better with new roads. Now, it uses those roads as a device for making money to fund the government. The M1 has become nothing more than a cash cow.

They say that they are widening the carriageways from London to Watford, and they probably are. But when work moves this slowly, it’s hard to be sure that they’re telling the truth. What we do know is that by putting cones on the hard shoulder, they can claim that roadworks are happening and this means, of course, they can impose extra low speed limits to protect the workforce.

How this is possible I don’t know, since the workforce is all in Dublin, drinking Guinness. But no matter. To enforce the 50mph limit, they have erected average speed cameras not just over a short stretch but for nearly 20 miles.

And so, onwards you trudge, at caveman speeds not daring to look up from your speedometer in case you accidentally do 53 for a while. This would then require some mental maths to work out how far you’d have to travel at 47 to bring your average down again. And since we know we can’t use a mobile phone while driving because it’s a distraction, we can be fairly certain, calculators aren’t allowed either.

It’s absurd. Plainly, the M1 is no longer what the politicians now insist on calling “fit for purpose”. Endlessly widening it means it’s endlessly narrower and even more useless than if they’d left it alone in the first place. They should give up and simply build another six-lane highway that runs in parallel.

There is time to think about all this, and exactly where you’d shoot objectors, as you crawl along, in a slow moving maths exam, with no one looking where they’re going, all the way to Watford, where the limit ends and everyone hits the loud pedal. Scientists say it is impossible to go faster than the speed of light. It isn’t. Not when you’ve just spent half an hour doing 50 and you’re late. Everyone, even people in Nissan people carriers, burst out of the cones doing 670,616,629.7 miles per hour. And away from Corby.

Like all sat nav systems, you can choose in a BMW the criteria for your journey. Do you want the fastest route or the shortest? Do you want to avoid motorways or toll roads? All good stuff. But plainly the unit in the M3 was jammed on a setting that took the car past as many forward-facing speed cameras as possible.

So once I’d turned off the M1 in Northampton, which is officially listed in the AA road map as being “nowhere near Corby”, I was faced with mile upon interminable mile of tedium in the face of Big Brother. And the car wasn’t much good either.

I like the new M3. As a coupé, it is a surprisingly elegant thing of understated charm. It’s fast too. Really fast. At the Ascari track in southern Spain last year, it was a full five seconds a lap quicker than the ever so shouty 6.2 litre Mercedes C-class.

Best of all, though, you no longer have to be a pushy oik to buy one. Today, people with Take That haircuts, Oakley sunglasses and short-sleeved shirts are to be found in fast Audis, leaving BMWs to people who simply want a fast, practical means of getting from A, via Dorset, Aberdeen and the Kamchatka peninsula, to B.

The saloon version is even better. It doesn’t have a carbon fibre roof, which makes no difference at all, but it does have four doors and a bigger boot, which means your children can come too. And it’s a little bit cheaper.

The new convertible version, however, has a problem. Taking the roof off, say, a Peugeot doesn’t really matter. Who cares if it’s all floppy as a result. It was never built to be the ultimate driving machine in the first place.

The problem is that BMW’s M cars are built to be the last word in precision, handling, fun, grip and speed. And if you take the roof off, you are sacrificing torsional rigidity, which means you are sacrificing precision, handling, fun, grip and speed. You are therefore removing the whole point of the car. It’s much the same story with the Porsche 911.

You can feel the floppiness as you drive along. There’s a vibration in the steering wheel and a sense that all is not quite right in the corners. You can feel the weight too. It feels all the time like you’re dragging an anchor.

It wouldn’t be so bad if BMW had stuck with a canvas roof, but due to perfectly sensible market demand, it’s gone instead for the folding metal option. It really doesn’t work because apart from the extra metalwork this requires, the styling is hopeless. To get the back window to fold into the boot, it doesn’t slope like it should. Instead it rises like a small cliff from the base of the boot. This allows the roof itself to split in two and stack underneath it before folding into the boot. It’s all very clever but you don’t half feel like a show-off if you press the button in public. And good though the Germans may be, you just know that, five years from now, it’s going to jam.

Other things? Well, although back seats are fitted, God has not yet made a creature that would fit in them, and with the roof stowed, the boot is useful only if you are a naturist.

So, to conclude. The new 4 litre V8: very good. Lovely. Nice noise. Lots of power and bags of torque, whatever that is. The ride: excellent. Here at last is proof – are you listening, Mercedes? – that a sporty car does not have to shake your eyeballs out of their sockets.

I also like the system that uses energy from the brakes to power the electrical appliances. This means the alternator has less to do and consequently takes less power from the engine. That’s good for fuel economy, slightly.

But there’s no getting away from the fact that if you want a convertible, you are better off with an Audi RS 4 or a Mercedes. And if you want an M3, you are better off with the coupé or saloon.

Just be aware. Until BMW sorts out the stupid sat nav system, you will also have to invest in a portable TomTom. Because if you rely on the idiot in the dash, you’re going to spend the rest of your life in Guildford, looking for Edinburgh Castle.

Vital statistics

Model BMW M3 convertible

Engine 3999cc, eight cylinders

Power 420bhp @ 8300rpm

Torque 295lb ft @ 3900rpm

Transmission Six-speed manual

Fuel 21mpg (combined cycle)

CO2 309g/km

Acceleration 0-62mph: 5.3sec

Top speed 155mph

Price £54,760

Road tax band G (£400 for 12 months)

On sale Now

Verdict Compromised and confused cabrio

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