We fill up the car about once a fortnight with 50 litres of diesel at a cost of about 55 euros and we do 800 km before filling up again. The price varies but everything else stays about the same.
Then last time I filled up I saw that they were selling super-diesel with super detergents and all sorts for 5c a litre more. So I thought I'd give it a go.
940 km later on the same tankful I think I'll continue the experiment for a few more weeks !