My Rubi is fairly heavy with winch, armor, bumpers and swing away with 33's.
It runs 450-600 miles a week mostly highway and most of that at 73 mph (speed limit 65). I get a little over 15 mpg.
The dummy light comes on at 234 miles and it will take about 15-15.5 gallons of gas at that point.
All mileage figure with a GPS since you cant just drop a speedo gear in a Rubi