07 Dodge 2500 4.30 gears

[Wheelin66bronco]

I just swapped to 4.30 gears in my 07 2500, previous was 3.73.
My question is on the speedo correction.
I run a smarty touch, which had ABS/speedo correction.
I'm running 37x12.50-18 Nitto Ridge Grapplers. With the 3.73's I had the smarty set to 35.5" tire to get correct speedo tested by GPS.
With the 4.30's I have to go all the way to a 27" tire in the smarty to get the speedo correct by GPS.
This seems way excessive to me, I figured it would be 32" which is OEM size.
Is the gear ratio a part of the ECU flash, or does it just do everything off the tone ring and tire size setting?

 

[akguy09]

Start with the stock sized tire and gear ratio, figure out what your new differences are and go from there

 

[Wheelin66bronco]

Had my buddy flash the ECU for 4.44 (no 4.30 option) and 11.5 tone ring.
Set at 35.5" tire, and I'm about .5mph off. Couple tweaks and should have it spot on.