The other day, I went to buy new hubcaps for my car.

The other day, I went to buy new hubcaps for my car. For some years I've been driving it around with an odd set, and a week ago one of these fell off. Normally I don't notice such things, but even I saw that it was probably time for new ones.

Where do you find hubcaps? In a car accessories shop, silly. The one I picked was called Auto-Passion (this was in France, where shops have names like that). It was full of wire wheels and nameless pieces of metal. There were four or five dudes in oily jeans and greasy hair hanging out by the counter.

When I asked for hubcaps - the word is enjoliveurs; literal translation: prettifiers - they laughed scornfully. "You need a supermarket, Madame," they said. Clearly (they implied) no serious parts store would dream of stocking so naff an item.

Of course, not all pieces of useless auto-decoration are petrolhead-unfriendly. Think of los flambes, the handpainted flames fashionable amongst Latinos in Los Angeles. Or saloon-car spoilers. Or that neon underlighting that makes your night-time car look as though it's just gliding along a foot above the road.

Whatever your taste in such items, they all have one thing in common: they are not necessary to the efficient functioning of the car. But we like the way they look. So we buy them - and, incidentally, the car they happen to adorn: thus once again demonstrating the truth of Alfred Sloan's great aperçu that what sells cars is not the engineering.

This, of course, was a marketing breakthrough as far as the car industry was concerned. When Henry Ford brought out the Model T, the market for popular cars seemed bottomless: everybody wanted one.

But comparatively soon everybody had one: so what then? Model Ts were built to last. And the cost of introducing a new model was frightening. It involved not only designing a new car but retooling the entire production line. Ford eventually bit the bullet: the last Model T appeared in 1926, after which everything closed down amid enormous secrecy until, in 1927, the Model A was introduced: an anticlimax from which, it could be argued, the Ford Motor Company has never entirely recovered.

How was the auto industry to survive the saturation of the first-time buyer market? The man who came up with the answer was General Motors' Alfred Sloan. Ford, like all the pioneers of motoring, had been an engineer, and the the Model T - tough, cheap, adaptable, easily maintained - was a triumph of imaginative engineering.

Sloan's background was different. He was an industrialist who had come to General Motors because it bought his ball-bearings company; and he realised that the problem was not one of engineering, but of marketing.

Once people had bought a car, they didn't need to buy another until it died. But needing is not the same as wanting. You don't need a new dress or coat every year. But you probably buy one, because last year's looks somehow wrong.

If the auto industry was to thrive, the car, Sloan realised, must become a fashion item.

Sloan made his great move in 1929. That year he hired Harley Earl, who had made his name designing extravagant, personalised cars for the Hollywood stars, and brought him to Detroit. Until then, what a car looked like had been dictated by engineering requirements.

But soon Earl's design studio, not the engineering department, was dictating appearance. If curved windscreens looked better, then windscreens curved, even though they were hard to make and distorted vision. They were pretty, and so people wanted them. And what people wanted, people bought.

And now my wheels look lovely in their new hubcaps. Thank you, Harley Earl.

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