After I set it , it gains speed continuosly. 1979 olds 98 ,6.6 Lt (403cu.in.) engine.

Dad says: There's a vacuum-operated cable that's attached to the speedometer cable. Have someone take it apart and clean it, and it might go back to working. My father's taken those apart and had to clean them, so I'd take his suggestion. If that doesn't fix it, I couldn't tell you what would.

My father's an all-knowing source for everything GM, so I take him pretty seriously. Hope this helps!