I have the flat spotting issues with mine as well. I too upgraded from XLT, 17" wheels but had the optional larger tires when new. My speedo didn't change at all because the total circumference didn't change that much. Larger wheel but shorter sidewall = about the same rolling distance. Although yours may be a little off due to the fact that you probably had the standard 17" tires with a slightly lower sidewall(aspect ratio).

 

I also have the 20" factory rims with those Pirelli's. They do flat spot more in the winter versus the summer. I used to run mine at 35-36 psi and it was really bad. Once I raised the pressure to 42-43 psi, the flat spotting dimished. It's still there, just less noticeable. It goes away much quicker once the tire warms up now.

Good luck.

Quack Addict Aka Mike

'06 SCREW FX4