My 2016 Sprinter is fully built out with lots of the extras most people posting in this thread are going to add. The reality of that is that you will top the GVWR and just like the waistline, weight management will always be an issue. Run properly inflated E rated tires, upgrade the suspension and enjoy, the Sprinter handles the weight well. (One of these days I will look into brake upgrades, but that's another thread for another time).
The 2wd Sprinter has a 26.4 gallon capacity, the 4wd Sprinter has a 24.6 gallon capacity.
The reserve indicator light goes on early, with approximately 5 gallons remaining (per the manual). I have tested this, I came up with 4-1/2 gallons so 5 is a good number to use. The reality is that no matter how you try to ignore the light, it turns on with just under a quarter of a tank displayed and fuel is on your mind when it is on.
I have consistently gotten around 16 mpg fully loaded, on and off road, using the D5 and the high idle. I have added a bit more weight recently (steel rock sliders vs. aluminum Nerf bars, side ladder with Hi-Lift jack attached to it, rear skid plate to protect XL storage, those sort of things). Our most recent solar eclipse trip was 2,509 miles across six western states and Flint delivered a calculated 15.72 mpg (NOT corrected for 285 tire size, I ignore that for daily use). To add a bit of a safety factor, I suggest using 15 mpg as a real-world number for one of these heavy vans.
24.6 gal x 15 mpg = 369 mile range
24.6 gal + 14 gal = 38.6 gal x 15 mpg = 579 mile range
For me, the appeal of the Aero 14 gallon fuel tank is that it fills from the rear wheel well, thus no penetration on the side of the van, and it is on the passenger side which makes for much better weight distribution. (Flint currently carries 53% of his 150S floor plan weight on the driver's side and 47% on the passenger's side).
I am waiting for responses to several questions from Aero (the long weekend has gotten in the way), see below for the few pictures I have to share.
14 Gallon Fuel Tank Dimensions. I believe the two holes shown on top (set in 4" on either end) are the mounting locations, I simply don't know. This is one of the questions I am waiting on clarification for.
14 Gallon Fuel Tank Fuel Transfer Pump. Fuel is pumped into the main tank via a driver-controlled switch when there is capacity available.
14 Gallon Fuel Tank Fill Nozzle (located in the rear wheel well). On my van with 285's I measure 6" clear between the tire and the van body, thus I see no reason why this won't work with the Van Compass 2" lift, maximum 315 tires size.
With all of us looking into the various options we will gather the information that will allow each of us to make the individual decision that will be right for our builds. Thanks for the help, looking for more!