Following BMW's success with stop-start systems, which can save significant fuel in traffic, Fiat will offer such a system as an option on 1.2-litre petrol versions of the Fiat 500 and Panda. It will arrive later on for diesels, as showcased in the diesel-fuelled 500 Aria concept car at Geneva which scored 98g/km in CO2 tests, but Fiat's head of prototype testing, Maurizio Barantini, says there are bigger gains to be made with petrol engines becausse diesels are more economical to begin with. He says the improvement with stop-start on the 1.2-litre engine is around four per cent, with the 1.3-litre turbodiesel around 2.5 per cent.