I don't think that poll is accurate. I got the same mpg as everyone else with a V6 when I had stock tires. Add bigger tires, racks, etc - that will cut down on mpg. Most people with 4x4s have that type of stuff vs 2wd vans that don't.
I average 15-16 mpg. 33" AT tires, 8k lbs, roof rack. I quit keeping track as the diesel heater screws with the mpg numbers, but on a long drive, I can plan on 15 consistently.
I have a Sprinter V6 4x4 with 245/75/16 AT tyres. Plus a completely standard 2wd VW Crafter 2.0TDI.
So same driver, same roads. Imperial mpg.
Sprinter 24-26mpg
Crafter 34-38mpg
I think the VW engine is marginally more economical than it's MB equivalent but you get the picture
I just completed a trip from New Messico to New Jersey and back.
On the the way East, I got a steady 17-17.5.
Returning, against some nasty head and side winds, 15-16.
Of note was lower operating temps than I normally see running around the Western mountains.
I never saw anything over about 208. Closer to home, I regularly get into the 220s.
In 6k miles so far on a 2018 Sprinter 4x4 with 265/75 R16 Falkan Wildpeak ATs, we've never seen less than 18.2 MPG. Average has been in the mid 18s, and best was just over 20.5. Hand calculated at every single fill up and corrected for tire size change. Mix of highway and stop and go driving.
just finished a 800 mile weekend trip in texas both ac systems running and windy both fill ups were identical at 14.7 mpg. this was combined highway a t 70 to 75 and city driving. the only way your getting more than 15 is driving like grandma and staying under 55.
I have a low top 144" and drive 65mph. I see 18.5 to 19 honestly. Admittedly my van is very lightly loaded, two dirtbikes and some camping gear usually max.