Find a section of freeway or whatever, that has measured markers on side of road. At 60 m.p.h with stopwatch should take a 60 sec.Then decide fast or slow. 55sec.=65mph,65sec.=55mph. The speedo gear is behind housing cable threads to on trans. Take bolt out and wiggle housing out. Count teeth,add teeth to slow speedo down,subtract to speed it up(trying to remember). Local GM dealer should have gears,maybe chart to get close. gooduck!