New Speedo, it reads high and low, I need your help!

[zenzone]

So my new speedo head reads high and low, this is so baffling to me. When going 30(by gps) it reads low around 22 or so and when going 65 (by gps) it reads 80. When going 50 (by gps) it reads the correct speed traveling.

So far I have installed;
A new speedo head
A new slightly longer speedo cable routed very smoothly
Tightened the yoke nut to spec
Checked the drive gear and it doesn’t appear to be spinning on output shaft.

I have an 18 tooth driven gear (yellow)
I have a white plastic driven gear (I assume the white means 6 tooth)
I have what I think are 4:11 gears with 33 inch tires.

What would cause the speedo to read high and low? Is this a calibration issue? I thought a brand new head from our vendors would come calibrated correctly and I would only have to change the driven gear for larger tires.

Thank you for your thoughts and time!
Zen

 

[Broncobowsher]

It's a linearity issue. Find yourself a good old school speedometer shop and have a talk with them.

What needs to happen is the Eddy current drive and the spring need tweaking. 20 years ago I had a mechanical tach (same mechanism as a speedometer) rebuilt by a local speedometer shop. He got it dialed in.

 

[mrdrnac]

I'd call the vendor of the speedometer head, I'm sure they will help you if it is one of our Bronco vendors!

 

[zenzone]

It's a linearity issue. Find yourself a good old school speedometer shop and have a talk with them.

What needs to happen is the Eddy current drive and the spring need tweaking. 20 years ago I had a mechanical tach (same mechanism as a speedometer) rebuilt by a local speedometer shop. He got it dialed in.

I’m thinking you are right. I just thought the new head would show up calibrated correctly as it said it was a ford licensed product.

I’ll call the vendor and see what they say Monday. I talked to them once already, the first speedo head I received had a burr on the bearing so it would get caught and not free spin.

 

[69_Sport]

You might consider a good used unit. You can bet the replacement is of Chinese origin. In days gone by, it made sense to buy new. Today's market is completely different. Whether it's tools, auto parts or most anything else, offshore=junk. Need I say more?