275/65R18 To New Size 305/55R20, Please Help

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I went on php and printed out there gryphon manual/instructions.
On pg. 15 is a calculator on how to do this. So I plugged in my stock tire info and got 2483 (this is with the pho squish factor of .97) I also plugged in my new size (305/55/20) and got a reading of 2570.44

So if my 20"s give me less squish factor should I multiply by .98 instead, would that be about 7% ?
This would give me a reading of 2596.94 (2597)

So one last time to sum it up guys, I go to my 87 performance tune, hit custom options, scroll to tire size and enter either 2570 or 2597 (depending on which is more accurate for squish) hit enter then scroll to gear ration and ensure it says 3:73 and then I'm done?

I realize this isn't done with a gps, but this will get me closer to accurate than what I have now right?