speedometer off and doesn't return to 0

Problem started this morning, speedo was at 25 when truck not moving, speedo still moves with accel and decel, a little jumpy at higher speeds, has been at 25 and 30 and 20 when truck is not moving, but still moves up with accel and down with decel. Speedo seems to stay 18 to 25 mph higher than truck moves judging by feel and following cars on the road, no issues with odometer, no warning lights on, no check engine light on. Truck runs fine, I can't find any instructions in my manuel or internet forums with this exact problem. How do I diagnose and fix the problem? I am a mechanic, this is my personal vehicle, and i want to avoid the dealership, Thank you for your time.