i have a 79 mustang with a c4 tranny and a 3.73 gear rear end and my speedo is off by 10 to 20 mph when im driving i know i to get something for it and i dont know if i have to go to a speed shop or a tranny shop

 

Josh, at the end of the speedometer cable where it goes into the trans is a plastic gear,,you need to change this piece with a different ratio inorder for the speedometer to match the actual speed of the car............

 

see just ask dad