Calibrating speedo

[69 GTX]

I have the speedo out of my car cleaning it up. I'm wondering if anyone has any tips on checking the accuracy of the speedo? I thought I could spin the drive with a cordless drill. But I'm looking for a correlation between MPH=RPM. I see it has a spring for adjustment and what I think is a reference tab for location.
BTW this is out of a 69 GTX.

 

[69 GTX]

You can see what I think is the factory reference tab right where the orange paint starts.

 

[Yatzee]

I have the speedo out of my car cleaning it up. I'm wondering if anyone has any tips on checking the accuracy of the speedo? I thought I could spin the drive with a cordless drill. But I'm looking for a correlation between MPH=RPM. I see it has a spring for adjustment and what I think is a reference tab for location.
BTW this is out of a 69 GTX.
View attachment 423821

You will need to confirm how far off from the actual speed you are. Following someone with a known good speedo in their car or using a smart phone app you can measure the actual speed against the speedo's reading. If is consistently off at 30, 50, and 70 MPH (say 5 mph ) the easiest way is to remove the pointer and replace it to indicate the difference. If it's way off perhaps a new speedo gear is required. Did you put new skins on your rims because this is the most common reason. I notice that when the needle is at rest position you are showing 5 mph indicating the needle is in the wrong position. And, oh ya, quit driving so fast that you're burying the needle.

 

[69 GTX]

The speedo is out of the car. It goes to zero in the vertical state. There is a spring which increases/decreases the resistance of the magnets. I'm aware of changing tranny gears.

 

[Dibbons]

This is a "science" that many of us would like to see a tutorial on.