I have a 1991 GMC topkick and the speedometer is stuck on 40 mph

Under your truck a the side of the transmission you will see a cable that goes into it. . the cable has a cap screw or a little fork with a bolt holding it in. remove cap or fork then pull out speedo unit and you will see a cable with a square at the end . With your fingers spin it and see if the cable spins freely, if so have a helper look at the speedo and see if the needle is moving when you spin. If not your cable is broken or stuck.If it does not spin stuck, lube it. if it spins but the speedo does not move replace the cable. Hope this helps. MIkel