From what I've read we have a capacity of 25 gallons. The fuel light came on, I drove about 10 more miles and filled up, but it only took 20.5 gallons.
Do we have the largest reserve tank under the E ever? It was just below the last line before E.

 

IIRC, the light comes on when there is 5 gallons left. Depending on whether the vehicle is on flat ground, angled up or downhill will effect how the fuel in the tank moves the level float.

Assuming everything else is working (filler neck tube, vent tube, the shutoff feature of the gas station pump) your level sender might only be off by .04% (1 gallon).

I have driven mine to and from my work (about 3 miles) up to six times with the low fuel light on and never put in more than 24 gallons when I make it to the station.

 

holy cow. So it really does just have the largest reserve volume of any vehicle ever. That's an insane amount of gas after the light

 

The reason being the reserve indicator light is for those who run close to the wind on fuel, it's a rough guide to the amount left in the tank. It's designed for the diesel numpties who want to run on an empty tank and then cannot get the truck started albeit a common rail and self priming. It also stops people draining their tanks and dragging through all the dregs and crap from the bottom of the tank into the fuel filter. Only once in 12 years have I ever run low enough for the light to illuminate on the fuel gauge as it's not worth the stress when in a hurry when cruising on the autoroute. Generally I always run all my trucks with a minimum of half a tank of fuel just in case of emergencies also you never know here when the delivery ah's will be on strike

 

Careful, running out of gas is a pain. Mine required 3 1/2 gallons to re-prime before the truck would start again. Not to mention the silly little fill hole with flapper.

 

For a vehicle that gets in the low teens for MPG, 5 gallons isn't exactly a hug reserve. Also as was mentioned it's an off road vehicle, so you might need a little extra in the tank if your aren't on flat ground.