F150 dash mpg doesn’t match real world. Why?

[hebelr]

Hi, I have just started using Fuelly and am very disappointed with its findings. My trucks dash estimates are way off the actual mpgs. I added larger street tires to try and change my axle ratio. My truck cam with real low gearing because it’s a heavy payload package. I then purchased a speedo recalculator to adjust my dash mpgs to be correct again. I know the odometer and my speedo are now correct because I checked it on a 200mile trip with gps. On my last fuelly entry my truck said 21mpgs for the whole tank, then fuelly said it actually was 18.7. I am just wondering if anyone else has had the same problem.