The drum appears to work fine but as you go to higher speeds it becomes inaccurate.

So at 40 MPH, the drum may say I am going 43 MPH, but at 65 MPH, the drum says I am going 80 MPH.

Is this just the nature of the beast due to age or what? No shimmies, strange movement all is smooth.

 

I don't know for certain this is your issue, but that is essentially what happens when the tire height or rear gear ratio are changed from the factory setup. The speedometer calibration will be off by some percentage, so it will read correctly at 0 MPH and as speed increases the MPH offset becomes larger. With shorter tires / lower gears the indicated MPH gets larger than the actual MPH; with taller tires / higher gear ratio the indicated MPH gets less than the actual MPH.

 

Thanks for the suggestion, I should have stated the car does have new Coker bias tires that are the correct factory size. So that should not be the issue and what was on the car when I got it was the same tires.

So that should not be the issue.

 

I think the speedo is run by a magnetic force from spinning, against a spring. Slower and jumpy means badly lubed cable or a hang up. Too fast, I presume, is a weak spring.

 

No bumpy movement, so it must be a weak spring. Can you replace the spring or get a NOS drum?


I will probably leave it but curious.

 

Hemmings Motor News lists several speedometer repair shops who can check and repair your speedometer .
https://www.hemmings.com/classifieds...et=Instruments
Speedometer repair and calibration is not a DIY proposition .