I reckon at least 15,000 miles a year. Diesel engines are usually 20 per cent - 30 per cent more economical than petrol engines. But the technology now applied to them to make them run smoothly with low emissions makes them very vulnerable to expensive failure that can cost far more than any saving in fuel costs. Diesel particulate filters last between 60,000 and 120,000 miles. Dual mass flywheels last 50,000 - 100,000 miles. Exhaust gas recirculation valves can fail. Turbos can fail. Repairs between years three and six could amount to £6000.