Does anyone know why, after putting in a replacement speedo, it goes straight to 20 mph?

I had a bad odometer, so I bought a used speedo...after putting the replacement speedo in, the speedo sits at 20 mph. It seems to be working when driving, but it's 20 mph too high!

Is there some sort of adjustment for this?

Thanks,

Ian

 

does it sit at 20 when not moving?

 

yes...cars parked, engine off...sits at 20. When I start driving it gradually increases (seemingly at a correct pace)