RE: 2009 R/T axle ratio
With the manual trans, you can opt for the 20 inch wheels, the 3.92 gears and the only way to get a limitted slip differential and the only way to completely disable the traction control. I know this from plenty of discussions with Dodge dealer sales and tech reps.
The 3.92 probably overcomes a bit of the "loss" of performance form the larger and presumably heavier 20 inch rims. The traction control on the other models can be disabled, but not completly, apparently in order to make up for the lack of a limitted slip, the traction control will possibly partially prevent the one-wheel-spin at launch.
The overall tire height diffence should not be enough to require a different ratio to match up the speedo for 18 or 20 inch rims. As indicated by Dodge Farve Ever, the difference is small, and most manufacturers try to keep the overall tire height difference within neglegible limits for ease of swapability (dealer upgrades).
(Removed references to Trak-Pak)