From reading up on the 1.21 firmware, the updates to it were supposed to correct speedometer calibrations for the F-body guys, and AEM recommends sticking with 1.19 for anyone else to keep things simple...Just a heads up. None of the maps for the SRT that I've seen use the 1.21 firmware, except yours.
Here's the info i dug up on AEMPower...
http://forum.aempower.com/forum/inde...c,19667.0.html