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Probably the gearing in the 6 speed is set for efficiency in the urban setting, or slower speeds rather than having a nice long 5th gear that sits at 1800rpm at 60mph.
This sounds very plausible @Scrabble
I do wonder if some of the Euro 6 emissions improvements have hindered performance on modern diesels. Sounds a bit gammony to suggest that, I have no evidence, just a thought. Wouldn't be surprised with some of the OEM behaviour.
NurseHolliday-
Probably a conspiracy theory but with diesel gate Audi supposedly did an AdBlue recall which negatively effected performance. Although I think car forums are full of nutters haha
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This is quite possible. When I think of the changes Citroën/Peugeot had to do to the 2.0 HDi engine to make it Euro 5 compliant, I don't suspect there would be a lot of room left to make it meet Euro 6 without reducing its performance in some way. In my Euro 5 Citroën it is 163bhp, and a good strong engine, but the newer BlueHDi versions are only 150bhp.
Haggis
bq
Dpf is definitely a possibility. I don't know if tyres could cause such a difference, but are they different new vs old van? Maybe the pressures or the brand are causing the drop in efficiency. Probably the gearing in the 6 speed is set for efficiency in the urban setting, or slower speeds rather than having a nice long 5th gear that sits at 1800rpm at 60mph.