The consumer organisation, Which? says the overall running costs of petrol cars may be lower than their diesel equivalents.

This year is set to be the first in which diesel cars will make up more than half the new car market. However, a Which? comparison of diesel and petrol versions of six popular car models has found that petrol engines can often be the more cost effective choice for drivers covering a typical annual mileage.

It says that diesel engines in the BMW 5 Series, Ford Fiesta, Vauxhall Astra, and Volkswagen Tiguan may deliver cheaper fuel bills than their petrol counterparts initially but it takes many years before they actually save the average driver money – up to 14 years in some cases.

The Which? study also considered reliability, taking information directly from the 2012 Which? Car Survey, which found that petrol cars are generally more reliable than diesels - both in the first three years of their life (the typical warranty period), and even more so between four and eight years-old.

We would agree with some of the above. For small cars like the Fiesta, petrol engines are now so economical that a diesel only makes sense for exceptionally high mileage drivers. However, very few buyers of a BMW 5 Series are ever going to choose a petrol engine. The so-called "Benefit in Kind" tax rules for company cars (and such cars are usually company cars), mean that a petrol 5 Series is vastly more expensive to run. The cross-over point for diesels is the Ford Focus/VW Golf class – that is where the argument is finely balanced between the two fuel types.

However, the point about reliability is significant. A diesel engine made since 2009/2010 which meets the current Euro V emissions standards is a phenomenally complex piece of engineering, with a trap to catch soot in the exhaust and a fuel injection system that NASA would be proud of. If one of those goes badly wrong in a few years time, it is going to cost an awful lot of money to fix.