Speedo reads half

[Zaairman]

So...ah yeah, just like the title says, my speedometer and odometer are now reading half of what they should. Happened today while I was driving home... was reading fine on the highway (70 MPH). Then when I got off, I noticed that it was reading 20 MPH when I was doing 40 MPH (verified on GPS). Got back on the highway, and it's still reading half. Clocked some distance using mile markers and GPS.... odometer is also reading half. Any ideas as to what it could be?