My speedometer seems to read okay below 25 MPH, but after that it reads slow. And on the highway I'm passing cars when it reads 45 MPH. I clocked it the other day and I was doing 57... And when it reads 55 I am doing more like 75MPH. Any ideas why? I changed the speedo head in my instrument cluster (put in a used one) but that did not cure the problem. (Note: My speedo is analog, but it gets it's signal from a speed sensor in the differential. I understand it's a bit difficult to replace this part so I was wondering if anyone knows what I should try first.

.... Can the Speedo be recallibrated without removing it? Or, might ther be an adjustment I can make?