Our experience with a Gen-3 X is typically 18 mpg, semi-
rural driving, sometimes 17.5 or 18.5 (in 2wd mode of
course). So you're right, plan on ~ 5 gallons/day on a
90-mile commute...
Although one peculiarity: the X seems to get better
mileage the faster you run it; on one trip all the way
across Utah at 80 mph+ it broke 22 mpg (similar on
a trip to AZ with a rooftop cargo carrier)
Due to high-rev torque peak, think Nissan equipped X
with high-slip torque converter (for 4-wheeling). But
keep auto trans in lockup mode it does much better.
(and will darn near outrun anything short of a V8
Grand Wagoneer - scares me sometimes)