Looking at ecoboost vans and it seems most have the 3.3.
Why is that?
My long 2 cents.....
It seems to me that Ford tries to match gearing to engine capabilities in order to please the largest number of buyers. They probably do lots of market research on what most people do and or want from their vehicles, but just like we see on this forum personal opinions, tastes, and needs vary considerably.
Fuel economy is probably important but since these vans don't have official EPA ratings it probably doesn't matter to them as much. Not saying it's trivial because the word gets out anyway.
On the other hand if fuel economy was the primary issue, it's possible Ford could install even taller gearing at the expense of other performance. But they don't because they also consider gradeability and towing (it's mentioned in their EcoBoost studies so it's at least considered). People don't generally want constant shifts because the engine can't pull the load even if it saves a little fuel (hence why CVTs are becoming more popular in small vehicles).
If the 2015 F-150 is any indication, Ford uses tallest gearing on engines with highest torque (over a wide range of RPMs). The base 3.5L V6 starts with 3.55, the 2.7L EB V6 and 5.0L V8 start with 3.31, and the 3.5L EB V6 with 3.15.
My guess is that they could also run 3.15 in the EB Transit, but on larger vans with much more aero drag than F-150 the gains would be insignificant. Once the engine has boost, adding more load through gearing does not make it more efficient. This is the reason why EB has a negative reputation for inefficiency when towing. Same can happen at very high speeds when using a lot of horsepower.
Ford papers I've seen suggest they plan to decrease EB displacement to improve fuel economy (like introduction of 2.7L EB) but that's only going to help at lower HP. At higher power levels (relative to size) most modern engines can be geared to be equally as efficient.
I personally think people like EB today mostly because of their power. Fuel economy isn't all that different in my opinion. Does anyone buy EB to save fuel? I kind of doubt it. If Ford continue to downsize EB displacement to equal power, then it will be interesting to see how buyers react.