I agree with smaller engine smaller car is effected more by a rack. Larger vehicles with larger engines not as effected. I think that pretty much sums it up. Here are some of my numbers and experience with it, I love roof racks and how much more useful they make vehicles. I have had a bunch of racks on lots of different vehicles. Here are some numbers of how it has effected mpg and noise.
Honda civic, gutless 4 cyl, rack with no bikes on, 2-5 mpg drop, some noise, not much, less noise with a wind deflector in front, more of a hum with no wind defelector.
Honda civic, same gutless 4 cyl, rack with 2 bikes on top, 4-8 mpg drop.
Nissan maxima, v6 good torque, 2 bikes on top, 2-4 mpg drop.
e350 5.4 v8, ujor 6" 35" tires, empty rack, less than 1 mpg drop, wind noise in front, and noise from rack metal on rack when hitting bumps. Have full expanded metal floor.
e350 5.4 v8, ujor 6" 35" tires, rack full of gear, 1-3 mpg drop, only 3 if have big wind deflecting things up there.
I cant offer much help on a sprinter as they are much different than a ford gas van. Seems like a large vehicle with a medium sized engine. I have talked to 2 people with sprinters and they said they noticed a 3-6 mpg difference depending on how much gear is up there.
How fast you drive I think makes a big difference too, I usually drive 10 over the speed limit but when Ive slowed it down that seems to help a fair amount also.