If the speedometer is in working order, I was told it needs to spin 1000 rpm for 60 mph. I am not sure what trans you have probably a 350 turbo or 700r4 you need the gearing to match your rear gears and tire diameter. There are calculators on the web. You may need only a diff speedo gear but it could be a housing and gear as Flxible said above.
http://www.tciauto.com/tc/speedometer-gear-calculator/As Carl and Wayne said above --check actual speed with a garmin, tomtom or a phone app for a sanity check.
I would guess you are now running a 4.10 gear and the trans was set up for a 2.73, 3.08, or 3.42 or so. If I am thinking correctly.
