Its the only thing that dissapointed me about my 150 dsg T6.
I run it completely empty and drive it like an old granny, but I can barely get over 400 miles to a tank. Each time it regens, ( usually twice a tank full ), it loses about 40 miles of range. ( 80 miles per tank ). This equates to about 30 mpg.
My 2015, 160ps Vivaro is my main site workhorse, carrys around 400kgs of kit constantly and returns around 38mpg.
Even my old T5 174 remapped to 200bhp carrying 400kgs managed an average of 33mpg.
I'm hoping the mpg improves with more milage, but I'm not holding my breath.