I have lubricated speedometer cables by removing them from the car and removing the inner cable - wipe lube all along the inner then put the inner back in properly.

Usually there is a knurled collar at each end of the cable - in older cars, these ar reasonably easy to access.

If you cannot or do not want to remove thee inner, you can hang the cable up by one end and applying lube to the top.

Tf you look at the inner cable. you can see that it has a twist to it. After you apply lube, spin the inner cable so that it pumps the lube down the outer sheath.

You want to see some lube come out the bottom.

This can be accomplished by giving the process adequate time for the lube to get to the bottom or repeating the lube and twist process.

There are a couple of youtube posts available - the only one that I viewed deals with newer cars.