[Jackpine]

Since you have the original Gryphon, you should have downloaded Pegasus. Pegasus has a "Gear Ratio Calculator" that can be used to determine your Tire Size (including squish). Click on "Calculators->Gear Ratio Calculator" to start this application. Now, enter your tire information this way: "305/55-20" (notice the "dash" before the 20) in the "Old Tire Size" box. The application is set up to apply a "rubber squish" value of 5% (you'll see that in the "squish" box). Once you've entered your tire's numbers, you'll see a value of 2596mm in the "Old Circumference box. Use that as a starting value - I think it's going to be nearly perfect and you'll note it is almost the same as the 2600mm figure Longshot gave you. (4 mm is less than 3/16 inch, by the way.)

Your 55-series tires don't really "squish as much as the more narrow aspect ratio ones. For those, I'd change the value in the Squish box to 7%.

As Longshot said, the truck's speedo and the Gryphon will always be different by the same amount. They're both getting the same information from the PCM, and the truck's speedo just has a "high side error".

You won't change your custom tune by correcting the Tire Size. Just open the "Custom Options menu" when given the chance and then scroll to Tire Size and enter it. Hit <Enter> to save it. Then, scroll to Gear Ratio and change that value to whatever your axle ratio is (if needed). You may not have to change anything there, but you need to check it. Again hit <Enter> when it's correct. Then scroll to "Done" and hit <Enter> to start programming.

- Jack