Have an 06' f350 6.0 The speedometer works fine most of the time. A couple times a week, it will read all over the board. +30, +50mph, ect. I hooked up my edge insight to read speed and when the factory speedometer reads incorrectly, the Edge reads correct. No codes. Any ideas?

 

Normally I would say this is a VSS issue, but the Edge reads the same VSS signal that the speedometer reads, so that would indicate the speedometer has issues.