Diesels as said can be economical but not as much as people think. It's assumed.
E.g. The BMW 530d isn't a good ok fuel as everyone thinks but it's quite fast for a diesel and doesn't feel like diesels of old. Also people never do their sums they have to add extra cost of car extra cost of fuel, extra cost of maintenance and more parts to go wrong. Turbos, injectors etc etc arent cheap and if diesels dont get hot urs not good for the engine your mpg isnt great.I've seen people sell good cars to buy new diesels thinking they are saving money because the tank lasts 50% longer but they spent 5-10k more on a new car.