I had the same exact "issue" two of my Tundras with the 38 gallon tanks. In the case of Toyota, this behavior is by design. From what I was able to gather, the instrumentation treats the tank as if it hold only 32 gallons.... essentially giving you 6 gallons of reserve beyond what the instrumentation/fuel gauge displays. In the case of Toyota, the intention of this design was to protect/preserve the fuel pump. Because of the shape of the fuel tank, when the fuel gets low the fuel pump can become unduly stressed. In other words, Toyota doesn't want you to drive the truck with less than 6 gallons in the tank . I would guess the same thing applies with the Raptor.