Those are referred to as Lie-O-Meters. Usually so far off it's not even funny. Hand calculate your mileage against what you put in at the pump. Then you'll have a fairly accurate idea of your mileage.
I have a 2011 F350 crew cab long box and my "Lie-O-Meter" is within .2 everytime I check it by hand. My truck has stock size tires and no lift, so it should be I guess.
For anyone wondering I average about 19mpg empty and have seen as high as 22. When towing my 8.5x25 foot enclosed trailer I have been seeing anywhere from 8-13 depending on how fast I go and what the wind is doing. I just went over 50,000 miles and it seems now that my mpg towing is getting better. I am consistently seeing 12 now.
