Volvo Repair: 1991 Volvo Sta Wag Speedometer, speed readings, speed signal

Question
The speedometer acts very erratically when I first drive  showing speeds of 69 mph when I' doing less than 30 and fluctuating rapidly up and down.  After about driving for about 15 or 20 minutes it seems to correct itself and provides accurate speed readings. What is the cause and can I correct it?

Answer
there are 2 possibilities the speed sensor in the rear axle or the speedometer head in the instrument cluster.the only way to be sure is to check each of those items when it is acting up.there is a volvo tool that can simulate a speed signal for the speedometer head and monitor the signal from the rear axle to determine the exact problem.