I'd register over at taurusowners.com for more info, but in a nutshell...
Miss on cylinder 6 could be an injector, the circuitry for the injector, in a worst case the ECU, or a plug/COP. I know you said you replaced it; however, I'd try swapping it over to #3 and seeing if the issue follows it, if it does its still a COP or plug, if not injector (which can also be swapped over to #3) or wiring, possibly ECU. Will need more info (like mileage) to make a more accurate guess.
Also get the exact codes and put those up.
As far as the speedo not working it could be the neutral safety switch (MLPS) or again wiring or connector, the VSS would also be high on the list of things to replace as it is common on Duratecs to go bad.
2011 Dodge Durango Crew 5.7L HEMI
2001 Volkswagen Beetle GLX 1.8T