Speedo suddenly 20mph off

[BgBlDodge]

This is on my buddy's 03-05 6.0. Said he was running 70mph and when he punched it the speedo suddenly hit the peg and then came back down and now it's reading 20mph off. Sitting at a stop light it reads 20 and as speed increases the needle moves correspondingly, just 20mph higher than actual speed. I'm thinking it's the speed sensor but I have no clue. He's got some sort of little programmer on it and there is no check engine light.

 

[Kleetus]

Had this happen to mine about a year ago, but it eventually went back to zero on it's own after a couple of drive cycles. No clue what it was, and haven't had it happen again.