Generally speaking, we assume that more powerful cars will use more fuel – after all, that extra power has to be generated by burning fuel. However, the fast pace of engine technology means that it is not quite so straightforward. A rather clever little website called www.cleanbhp.co.uk has worked out the amount of CO2 each car produces to generate each bhp – and the results are a real eye-opener.Some of the latest-generation petrol engines produce more than twice the power for the same CO2 or fuel consumption as older designs. For example, the Citroen C3 1.1 produces 139 g/km of CO2 in return for a measly 61 bhp, whereas the Mini Cooper S with 136 g/km of CO2 produces 181 bhp. Ironically, the two engines are even distantly related – BMW’s 1.6 litre engine in the Mini was originally jointly developed with Peugeot, owners of Citroen. So how can there be such a big difference?

The answer is that the 1.1 engine in the C3 dates back to 1972, whereas the Mini engine is state-of-the-art. All that stuff in the brochures about 16 valves, variable valve timing and turbocharging is not just marketing hype. Car engines have not progressed quite as fast as computers over the last 40 years, but they have come a long way. Anyone driving an automotive fossil is going to pay, both at the pumps and on the open road.

So can you just assume that any car brand with a high-tech image will have a range of shiny new engines? Not necessarily. Even VW has a few old nails left in its range.  Thankfully the 1974-vintage (although it felt more like 1874) Beetle 1.4 litre 8 valve engine died last year, along with its disgraceful 171 g/km of CO2 for 74 bhp.  However, the old VW Polo 1.2 S carries on with C3-esque 128 g/km of CO2 for just 59 bhp.  If you upgrade to the modern Polo 1.2 TSI, CO2 emissions drop to 124 g/km and power shoots up to 104 bhp.

The one manufacturer which has exclusively modern engines is BMW.  BMW is proud of the fact that engine is literally its middle name (Motor in German means Engine), and it relishes any challenge related to engine efficiency. Pretty well all its cars are either at, or close to, the top of their respective classes. For those not committed to the cars with the blue-and-white roundel, it will pay to look closely at the figures for each version of any given model. A little label like TSI for VW, or EcoBoost for Ford, can make an astonishing difference to the efficiency of your next new car.