Depends on manufacturer, my old picasso did mid 50’s on a run but my 65 plate 308 SW bluehdi would easily average 75+ on a run unlike the VAG lump I’ve just got rid of that managed to average 30mpg over 43,000 miles with 80% on the motorway, it’s all down to spending money on development and we know that the VAG group did no R & D on their diesel engines for a decade and instead cheated with software to try and save millions.