Ok where does the speedo cable enter the cabin on your Fairlanes? Mine is thru a hole in the floorboard about 12" forward of the left (when you're sitting in the car) front seat mounting location. Then it runs under the carpet up to under the dash.
I'm wondering if it should enter the cabin more forward, like maybe 6-12" under where the steering column passes thru. Might be less constriction and curves that way?
Apples to Oranges....
My 66 Galaxie the speedo cable enters thru an oval hole just to the left of,
and slightly behind, where your right heel would be with that foot on the gas pedal.
There is a "channel" depression in the floor and toe board to accommodate the cable,
as it makes its way up the toe board to behind the dash.
The depression channel obviously is to let the carpet lie flat.