A failing RCM can cause in a long list of issues to include a malfunctioning speedometer. The easiest thing to verify whether there are any issues with the RCM is to switch it out with a known good unit. If you know someone else with a Crossfire nearby see if they will let you briefly try their RCM in your Crossfire. Otherwise, do a search on RCM repair and look into whether the individual relays are functioning properly. For example, you might have poor solder joints, dirty relay contacts or a bad coil on your traction control relay.