With the prices of classic cars apparently immune to the ill-effects of a flat-lining economy and all capital gains derived from them being equally immune to the enquiries of the tax man, I am asked with increasing frequency how to spot a car to which classic status will one day become appended.

The fact that a car is old is neither here nor there. Like fine wines good cars can be improved by time but only when sufficient years have passed for them to no longer suffer any meaningful comparison to machinery made today. By contrast a bad car was, is and will always remain so. In fact it’ll probably get worse.

There are in fact many factors that decide whether a car will become a classic. A good brand is seriously useful on the quest for classic status. Beauty is helpful but not essential; raw speed is far less important than many think while any kind of successful competition heritage will massively increase a car’s chances of becoming a classic. Being fabulous to drive is a huge asset but even that is not vital as anyone who’s seen Citroen DS prices of late will tell you. Perhaps the most powerful determinant is scarcity: no car is going to become an appreciating classic while millions still roam the earth. Above all and for any combination of reasons outlined above, it must be interesting for reasons other than its age.

What to look for? I think there is a window of time in which a car will usually become a classic. Of course some cars make the grade when they’re very young, but they tend to be ultra expensive exotica; on the other hand I’d say that if a car has got to its 40th birthday without being recognised as a classic, there’s probably a good reason for that.

This means that the majority of so-called ‘sleeper classics’ likely to be recognised as such any time soon are cars built in the 1980s and 1990s. There are plenty of examples but I’m just going to name just one. So far as I can see, the Alfa Romeo SZ of 1989-’91 has been almost entirely missed by the market.

It is true that this many-eyed monster does not tick all the boxes – it never did anything in competition and it’s not exactly a classic beauty – but it is one of the most visually striking cars you’ll ever seen and truly fascinating. Based on a cut down Alfa 75 chassis, it provided a 210bhp 3-litre V6 driving the rear wheels alone through a five speed gearbox. It was built by Zagato and bodied in plastic so it weighed just 1250kg. You’d be happy to play the soundtrack of its engine at your funeral but at the time its real selling point was its handling: motoring journalists of the day struggled to find the words to describe the beauty of its balance and the lucidity of the driving experience it provided. I know, I was one of them.

Photo by Tony Harrison

But today even this is not its most compelling strength. It turned out that Alfa built just 1032 examples of this bizarre looking machine, making it rarer than a Ferrari F40. There are around 40 still registered in the UK yet you see them advertised for little more than £20,000, now the kind of money nice but standard examples of BMW’s E30 M3 are fetching of which around ten times as many were built. True, the BMW can play the competition card, but as a thing either to look at or drive the Alfa is special in a way the M3, fine car though it is, simply is not. Were I a smart boy I’d have bought one just before writing this.

So that’s my bet for a future classic. What’s yours?