Modern diesels have: EGR valves, Diesel Particulate Filters, Dual Mass Flywheels, Inlet manifold swirl flaps, Turbochargers and Intercoolers, High pressure Injection Pumps, Highly sensitive Injectors, Many have timing belts (including this Renault). There is simply too much to go wrong and if they are driven short distances from cold a lot will go wrong. The cost of this is far greater than the saving on fuel over a petrol engine, which is far less complex and far less likely to go wrong. Also the real life (as opposed to textbook) mpg difference is more like 45mpg v/s 55mpg than 40mpg v/s 60.