The Toyota hybrid is hailed as an eco-paragon, so how does it fare against a big BMW? To find out our correspondents go on a run to Geneva

But are transport and tax planners – here and in the US – being fair to the people who drive conventional cars? The official fuel consumption figure for the Prius – supplied by Toyota itself – is 65.7mpg in mixed motoring. That’s a claim not supported by many of the letter writers to The Sunday Times who say they get nearer to 50mpg. If our readers are right and the official figure is wrong it has important implications, not least of which is that people driving frugal diesels are getting a raw deal.

To find out we set a challenge: to drive a Prius to Geneva using motorways and town driving. The direct route is 460 miles but we drove almost 100 miles further to give the Prius the advantage of running in urban conditions where its petrol-electric drivetrain comes into its own.

We took along a conventionally powered car – a diesel BMW executive saloon – for comparison and drove both cars an identical number of miles (545).

Yes, Diesels get a raw deal, and in the USA we get down right screwed (BMW has no plans to sell any 4 cylinder diesels in the USA)