Variance between Gryphon and actual milage

[p40whk]

I've always been in the habit of checking my mileage whenever I fill up my tank and just got my Gryphon programmer a couple of weeks ago and am using the average and instant economy screens. I have noticed that the average economy for my Gryphon reads almost 2 MPG higher than what I actually get by dividing total gallons used by miles driven. I've run

I don't have any custom tunes yet so I'm running the canned 87 tow tune since I have an AFE CAI. My truck originally came with 235/70-17 tires and I just replaced them with 255/65-17 which is an option OEM size.

I am wondering if I need to set the tire size in my Gryphon or if my odometer is not displaying correctly. I've run 4 tanks though the truck and the results are pretty consistent.

Any suggestions?