Changing tire size to correct speedo not working!!!

[throttle out]

I'm trying to correct the speedometer on my 99 silverado. When I select the tire size and confim it it doesn't keep it.
Any help would be appreciated
Thanks!

[06MonteSS]

after set the tire size, you are reinstalling the tune, right?

so then yes it will be changed..

when you go back in to the tire size setting on the tool, it will NOT show you what it is actually set to, it just shows a default value.. it's not what it is actually set to..

confirm your adjustment by comparing your speedo to a gps speed.. if it's within 1-3mph of each other, then you're fine and good to go.

[throttle out]

So it will just always default to 36"?
If I change it to 33 and hit the green check I can go back in immediately after and it's back to
36 again.
If I change to 33 and install the tune I see no difference in the response of my speedo vs
Gps on my phone. This just adds to the lack of confidence.

[mikel]

so even if you make a large adjustment it doesnt affect the speedo?