You probably need one or two less teeth to make that sort of change.
Yes. 19 teeth. I’ve had the same set up almost 40 years
Ok. But why now? Tire size etc has not changed.
Why is not my problem. Results are results.
Any chance my after market tach is the problem? I have had it for years without issue, but I did read on a different thread that another owner with this speedometer accuracy problem found his tach was connected to the wrong post of the coil.
Not on your Cougar. The speedometer is purely mechanical. Since the problem showed up when you changed gears, it’s most likely you’ve got the wrong gear.
To clarify - the accuracy problem did not show up when I replaced the driven gear. . The speedo was not accurate before I replaced the driven gear.
So there were two issues before I replaced the speedometer cable and driven gear - inaccurate mph and bouncing needle.
So when you say you’ve had the same setup (assume that means the same 19 tooth driven gear, rear end gear ratio, and tire size) for 40 yrs, has the speedo been inaccurate that entire time, or did it slowly change over time?
Same 225 70/R14 BFGoodrich radial TA’s for years. Same 3.0 rear end. Same FMX transmission. Last week was the first time I EVER even saw a driven gear. Same 19 teeth, which is exactly what the driven gear calculator calls for. I Bought the car in 1979. When the needle started to bounce in the last year or more, the accuracy became questionable.
So, prolly something wrong with the speedometer itself. I have NO IDEA how speed is transmitted to and displayed by the needle. I have read reference to a magnet, a friction disc, etc. I guess I need to take it apart and see what’s up
I took it to a speedometer rebuild tech and he said the unit needs to be rebuilt. Estimate was $114.
I’m impressed that you even found a speedometer rebuild tech. Sounds like a bargain to me!
Didn’t have my glasses on, 
. Estimate was $314