My brother (a mod here I think) pointed me to a thread here: https://ohptools.com/blogs/news/how-to-calculate-your-tire-size-using-the-ford-as-built-tire-calculator-and-configure-it-using-forscan
And after I bought the forscan tool and he and I used my laptop. Its within 1/2 MPH now, it also...
Here is mine. Raptor grille, lift kit, Aftermarket wheels and tires. Aftermarket step rails. I also did the speedo fix for the big tires ! The canopy and bed liner are ordered and due in 3 weeks.