Yea, Lars does have a tendency to reduce it all to "essentials" *boring*.
His value is a good starting point, but I think mine is better!
DO use a GPS if you have one. It will give you an accurate readout of your speed. Get the Gryphon to agree with it by changing your TS as needed. Your dash speedo will then read about 2 mph faster, because Ford set it that way.
I suspect LT tires may have less squish, so the circumference value could be larger. The user manual calculation assumes zero squish. If you enter that value, your TS will be too big.
- Jack