Sprinter vs Transit big vans?
 

My friend is looking for a new van, he would like the extra high van for working or living, and long ones, but there seems to be very little info on the mpg differences.

Any ideas

Both are IMO aero wise as bad.
Smallest engine tallest gear ratios.
But I will soon test a aerokit parts on sprinter so it migth be better pick in that sense. But anyone can make similar mods to both vehicles.

' extra-high'
 

At the local airport I visited hangers for 'millionaire' corporate jets.
All the planes I got to tour had 'sit-down' seating, and maximum headroom was limited only to the center 'aisles.'
I figured, if 'the smartest people in the room' won't waste their fortunes on an aircraft you could 'stand inside', they might be the kind of person that that would travel in a VIXEN motor home, with a pop-up roof.
Maybe your friend's a 'billionaire', with disposable income so great, that 'cost' is 'meaningless'.
Typically, 'BIG Cd' , and 'BIG frontal area' is your enemy!

The new post office fleet will all be full standing height inside because even millions of extra falling of gas cost less than a few workman's comp back and head injuries.

Full standing height for who? @6'2" I always had to stoop inside passenger vans.

The advantage of the sprinter or transit is that it uses less fuel than dragging the typical box camping trailer and doesnt have backing difficulty.

We went with the promaster. It came down to a price/feature/reliability tradeoff.

Obviously a transit would have been ideal, but they are significantly more expensive. I mean you do "get what you pay for", but what we paid for should be more than good enough for our usage. We're only using our van recreationally and won't be putting more than 2-3k on it per year. With that level of use, the 2017 promaster with 75k that we bought will last us basically forever. We only paid $18.5k for it - a high roof transit in that price range would have had a lot more miles or other significant issues.

Same goes for the sprinter, with the added issue of maintenance. If anything did break, it would have cost a mint to fix. We can get the promaster repaired pretty much anywhere.

It wasn't a dealbreaker, but the promaster was also the easiest to drive. I mean none of them are arduous (I've driven plenty of larger vehicles), but the short OAL, FWD, and tight turning circle make the promaster downright pleasant to drive and maneuver, even in town.