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.
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.