The fuel gauge on our E450 is quite consistent but not linear, especially above 3/4 full. At 1/4 full, it consistently takes 42 gallons to fill. Exactly 3/4 of 55 gallons is 41.25 gallons. That's close enough for me.
My practice is to refill the tank before it goes below 1/4. No matter if the gauge is dead accurate or not, I worry about problems like an unexpected detour or being unable to get into the only gas station for miles.
I once checked a small gas station in the mountains on Google Earth to be sure I could get in it. When I arrived scaffolding blocked the side of the pumps that I needed. Fortunately, I was able to wind around the dirt back streets to get to the other side, and then do the same getting back to my direction of travel.
2011 Sunseeker 3100
1997 Pathfinder Toad