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?