One might expect that the current recession would not be kind to sales of diesels. 

If buyers are downsizing and looking for cheaper cars, they might be less interested in diesels, given that (a) smaller cars always have a lower proportion of diesel sales, (b) diesel models are nearly always more expensive to buy than equivalent petrol models and (c) diesel fuel is more expensive than petrol. 

However, the diesel percentage is soaring across all segments, especially where diesel has traditionally been weak (see table). 

In the supermini segment, made up of cars traditionally too small and too economical to need diesel engines, penetration has jumped from 14.8% to 19.3% in the past year.

At the other end of the spectrum, the luxury sports cars sector has seen diesel share almost double from 10.9% to 21.1%. With Jaguar now readying a 3.0-litre diesel XK8, that figure is set to increase further. 

It seems that the prospect of lower road tax (and ever widening differentials for higher CO2 vehicles) is focusing the mind of the consumer.

In some segments, diesel is now effectively mandatory: at the recent launch of the 7 Series, BMW said 2009 sales would be 85% 730 diesel, with around 10% of sales for the plutocrat-spec 750i and just 5% for the 740i (which BMW defined as the “diesel avoider’s” model). 

Even in the lower-medium segment, diesel is close to 50% for 2008 – and indeed passed 50% in November. That means, with the exception of sports cars, every segment above supermini is now predominantly diesel. 

However, as the old saying goes, “past performance is no guide to the future”.

With the Euro V emissions standard being phased in during 2009/2010 and the ferocious Euro VI emission standards due in 2014, car companies are preparing for a swing back to petrol. 

The costs of meeting these standards for diesel engines means that small turbocharged petrol engines are the future for lower medium models and below.

Today, the most popular Golf is the 1.9-litre diesel, which accounts for 47% of all Golf sales. That proportion is not expected to stay so high – after all, VW would not have spent a fortune developing the 1.4-litre TFSi if it expected it to remain at just 13% of Golf sales. 

Diesel is enjoying its moment in the sun and may well pass 50% penetration. However, don’t count petrol out yet – it will get more efficient and will enjoy a steadily widening price advantage over diesel.