##mitcaris.jpg --Right##IT'S a safe bet that carbon dioxide emissions performance will be the bedrock upon which revisions to the vehicle excise duty system are to be based next autumn. And, assuming that the latest Government U-turn talk has a solid foundation, diesel will enjoy a clean sweep of the fleet workhorse class in terms of fiscal friendliness, as company car parc oil-burners tend to spew out fewer CO2 grammes per kilometre than their petrol counterparts.

Roughly speaking, the more miles per gallon, the better the CO2 performance. Thus a Ford Mondeo 1.8i 5-door with 15in tyres and combined fuel economy of 36.2mpg is listed at 185g/km in the latest (July 1999) official New Car Fuel Consumption and Emissions Figures booklet. A similarly-shod Mondeo 1.8 turbodiesel with 43.5mpg produces 177g/km. New technology diesels are even better. The common rail Peugeot 406 HDi 90 saloon, for instance, manages 50.4mpg with 154g/km, compared with 33.6mpg and 198g/km for the 406 1.8-litre petrol.

Current educated guesswork suggests it is unlikely that cleaner burning, refined diesels like PSA's HDi and Volkswagen Group's TDI pumpe duse will suffer an additional local pollutants penalty. Good news for the diesel camp, and a clear enough cost-saving message to fleet managers. But what about those with drivers who insist on petrol power and who refuse to downgrade to a supermini to make the CO2 grade? The compromise is our long term Mitsubishi Carisma 1.8 GDi Sport: 41.5mpg and 164g/km. It may not be as fuel-sipping nor as CO2 efficient as a modern diesel, but it is a long way ahead of rivals in its petrol class - consistently a CAP Monitor upper medium 1.6-1.9 petrol leader.

It's quickish, quiet (with the sunroof blind closed) and comfortable, and hasn't stepped out of line mechanically in its 2,000-plus miles on our fleet. The only minor problems have been related to the Cobra remote central locking/ alarm system (it refused temporarily to lock the doors after a particularly heavy downpour) and the satellite navigation system needs a CD-map upgrade if it is to stop becoming confused on the new section of the A1 in Cambridgeshire. Meanwhile, the GDi's petrol consumption average is creeping closer to the advertised 41.5mpg, largely as a result of this driver having been sent to Coventry twice in one week.

Gervais Seymour