ONCE, the petrol versus diesel issue was simple: a gallon of diesel went further than a gallon of petrol. In general, it still does, but the issue now is whether that gallon of diesel is doing more to harm health and the environment than petrol. Don't look to the Government for a simple answer, because it hasn't got one.

Official statistics show diesel cars produce fewer climate-changing gas emissions - particularly carbon dioxide - than petrol counterparts. But the Government's recently published consultation document for the reform of vehicle excise duty claims diesel-fuelled cars emit 10 times the potentially health-hazardous smoke particulates and up to twice the amount of oxides of nitrogen.

It's on the basis of the latter comparison that diesel is done no favours at the pumps and is expected to attract a 10% higher charge than petrol when a new VED system is announced in the March 1999 Budget. With taxation and health scares conspiring against the fuel it's hardly surprising annual fleet diesel car sales have been falling consistently since 1994.

That's adding to the dilemma. Smoke-belching or not, diesel is more economical than petrol or gas-power, therefore lighter on CO2 emissions. Increasing the numbers of diesels in Europe appears to be the quickest route to achieving the lower carbon dioxide levels required of the UK and Europe by the Kyoto summit. Confused? Who isn't? Citroen and Peugeot aren't, for a start. They're firmly behind diesel and are putting even more faith in oil burners as the future of motoring with new, super-efficient common rail technology.