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

[sblue]

So like my title says I made a switch from my stock 275/65R18 to a new wheel/tire size of 305/55R20.

After doing a bit of driving, it appears that nothing has changed from my speedo to my Gryphons speed? My speedo has always read 1mph faster than my Gryphon up until about 65mph then my speed is about 2mph faster than the gryphon.
I got the same exact readings after my tire change. I though my speedometer would be slower with a tire roughly 1 inch taller?

Am I supposed to adjust the gryphon first and then compare?

What about tire squish, does that change with a bigger wheel? (20")


Is there anyone here that has made this switch and re adjusted your gryphon?


I'm horrible with numbers and I'm hoping someone here can give me a correct number to input into my gryphon.

From the tire calculators I tries I came up with the stock size at 32.07" and the new size at 33.20". This i believe is unloaded without squish.

I have a feeling even though my gryphon and speedo haven't change that I still need to make some kind of correction.

Many many thanks guys!

Jack&Janet? You out there? I know your the guru's around here and could probably just wave a wand and give me the right size to input. Lol

Again, thanks guys!