SPECULATION that that the new generation of frugal and economic petrol engines will start to challenge diesel’s dominant fleet market position in the years ahead has come from auction giant BCA. Although the average price of a diesel car always outperforms that of a similar average petrol model, BCA says in terms of retained value compared to manufacturer’s retail price, fleet diesel cars average around 36%, compared to nearly 39% for petrol cars. Add in the new model premium for diesel cars and the higher cost of fuel and financial claims for the benefits of petrol-engined models can be made. However, Euro6 emissions standards are coming (2014 and 15) and evidence suggests that the required engine changes could impact on the price of diesel more than petrol cars. Additionally, benefit-in-kind tax changes due in April 2016 favour diesel. BCA has started a debate, and used car market performance undoubtedly impacts on decision-making. But it is only one issue within the total cost of ownership argument and individual fleet decisions can only be made on the basis of financial modelling.