Just bought a 1979 Sport coupe. Speedometer doesnt work. Vehicle runs fine, tach works, but speedometer, does not. Where would I start on this

Mechanical background = Zero (minus motorcycles). But willing to learn and open, and learning Youtube quickly.

 

Disconnect the cable from the trans and swing it out to the side. Spin the cable with your fingers and watch the speedo to see if it moves a tiny bit. If it does the cable and speedo are fine, and it's the gear in the tail housing. If it doesn't move you can put a drill on the end of the cable and spin it clockwise a bit faster to see if it starts moving then.
If it doesn't move you have to pull the cable from the back of the speedo and check it to see if it's broken internally. Then replace if needed.

 

Awesome. Thank you. I'm excited this will be first fix on the car . I will do that today.