My 91 Chevy S-10 Blazer’s speedometer is about 30 miles off any way to fix it my self being there is no speedometer cable behind the instrument cluster just a electrical connection.
It could be the speed sensor in the transmission… With these electronic marvels, you replace parts until it works right, if it ever works right…
Maybe this will this site help
http://www.fixya.com/search/p695459-chevrolet_1991_s_10_blazer/speedometer