'WORKING here at CAP, we discover a lot about the dynamics of new and used car choice. Take, for example, the common assumption that the favoured engines in sports models are the big powerful ones.

Take the engine options on BMW Z4. Three are on offer – 2.2-litre, 2.5-litre and 3.0-litre petrol engines. It might be assumed that the middle power unit would be the most popular.

However, it is the 2.2-litre that accounts for 42% of all sales in the UK, with the 2.5-litre at 24% and the 3.0-litre accounting for 34%. It is still early days for the Z4 but research is revealing that, of the few so far reaching the used market, the smaller engine is again the most sought after.

To add weight to the theory, we can look at the outgoing Mercedes-Benz SLK, which now has some history on the used market. Here, the new model split for last year was 200 Kompressor – with 45% share – a similar percentage for the 230 Kompressor, with the 320 and 3.2 AMG trailing with just a few per cent each.

On the used market this public taste is replicated, with the smaller engines again more popular. While it might be tempting to suggest that this is simply because they are more widely available, research in the used market demonstrates that customers will generally refuse to be talked into the larger engine varieties even when their first choice is unavailable.

In this instance, image, economy and comfort are far higher priorities than performance. This mirrors our findings in recent years for all used cars. In the secondhand market, big thirsty engines are rarely favoured by the customer.

On the same theme, information from Audi about the model mix for A8 also reveals a similar pattern.

There are six engines now available, ranging from a 3.0-litre petrol to a 6.0-litre W12 petrol unit. In this prestige market, with high upfront purchase costs, again one might expect the buyer to choose the smoothest and fastest engine. But with diesel technology having moved so far forward, acceptable performance and smoothness are now available in combination with economy.

Therefore the new sales breakdown projections by Audi show a projected 50% share for the 3.0 TDI version alone. Compare this with the second place position of the 3.0 V6 petrol, trailing at just 18%.

Furthermore, when it comes to the really big power units, none of these reach double percentage figures.

What this shows us – and anyone with an interest in the future used market fortunes of these cars – is the desirability of the most economical versions of most cars in tomorrow's used market.

This is reflected in CAP Monitor forecasts which, in the case of the Audi, project a used value at three years and 60,000 miles of 29% of list for the 6.0-litre and 39% for the popular 3.0 diesel.'

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