Maybe I can help clear the mystery. I had a 2011 Ram Outdoorsman with the 32gal. (as you already know 26 is standard). I had the same issue and was confused. Light comes on, I drive a few extra miles and still tops out at 25.5 gallons. So I did the unthinkable...I ran it dry. I had a gallon for my lawn mower in the bed; got it started and off to the gas station. 32.2 gallons. It was cold out and I figured the fuel condensed slightly.
Many of the concerns of doing this is real. 1. If your area has suspect gas, avoid doing this. 2. If you travel and frequent unfamiliar stations, avoid doing this. 3. If you live in dusty/desert environments don't do this. Impurities, sediments, etc. will settle to the bottom of your tank and guess what gets sent to truck's heart when you get past the last couple gallons? YES, it starts eating the 'yuck yuck'. I was very diligent on my truck and its services. It NEVER failed me. I ran it dry twice. Once for this experiment and once because of bad luck. Older vehicles in general may have pump issues, seal issues, or priming issues. If there is a history of this in your vehicle line then use more caution. 4th gen RAMs don't seem to have these issues but I can't guarantee anything.
So as soon as my light goes on now, I hit the reset on the trip odometer and I know I had another 100 miles to go. I will fill up about 29-30 gallons. That is based on MY vehicle and MY driving habits. I hit a 100K miles on that baby and traded in for a Rebel with a 32 gal tank so I will play a bit and provide some numbers later when I have them.
Today, my light went on for the first time on my Rebel. I immediately pulled and filled. 25.5 gallons. Dash stated I had another 50 miles. Indicators are usually conservative and probably calculated at 12.5 mpg which makes it 4 gallons. Leaving another 2.5 gallon 'reserve'. I get about 15 mpg city. So I estimate from the point my light goes on I actually have 90-100 miles before I whip my phone for AAA. Oh wait, I don't have AAA. Better not test this theory just yet. But based on my experience, the code (NFX), and indicators, my newly acquired Rebel indeed has the 32 gallon.
The 32 gallon is my single most favorite option. It should be standard across the fleet.