Speedometer calibration/adjustment...

[Army_Dude]

I was lucky enough to score a used speedo from a board member after I broke mine. I got it in last night and when I drove it today I notice it is WAY off. It jumps up to about 20 mph almost instantly and it reads 70 to 75 mph when going down a road I know I travel 50 mph at best. Is there a way to adjust/calibrate or do you just live with what ya got? OR>>>>Is it possible I did something wrong on the install, it seemed pretty straightforward but hey I had to ask (ya never know). Thanks!