The drive was over the same path and same distance, however, the last one-half was uphill, with considerable elevation gain.
The Grenadier's fuel gauge reads high almost exactly to the same extent as old D1/RRC - when it shows 5/8 of a tank, it's about half a gallon to a gallon under one-half. It is moderately useful; I found that on the same routes I used to drive my older trucks, Grenadier is more fuel-efficient, including ~200-mile, 3-day Death Valley crossings. But in places I am not familiar with, I should definitely have a full jerry can along.
My guess for the Grenadier's funny remaining range estimates is short integration time, coupled with high variation in fuel consumption of the engine with variable displacement (turbo/supercharged). I'll ask my daughter and son-in-law if they found the D5's range estimate reliable.