I would make a drastic exaggerated change change to the tire setting and see if it has an effect on your truck speedo.
If it does then it's probably fine tuning the value.
If it does not change the truck speedo it is probably not writing value correctly.
Bill will be able to help you either way but it will save you both time to verify where the issue lies.
BTW the truck speedo will always be a little off (2-3 MPH) but the Gryphon and GPS should be right on.
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