I have a newly rebuilt VDO speedometert hat is unsteady over 20 mph and jumps when I hit a bump. The company bench tested it and said it's okay, but I'm still having problems. Before I go back to them can any of you think of what it might be? Maybe something external? It's connected to a new German-made cable that makes a tight S-curve between the bulkhead and gauge.Could that be an issue?
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The S-curve, if too tight, could be the issue. Get some liquid graphite, unscrew the cable from the speedo and dump some down in the cable sleeve. Reattach and see if it's any better.
Come to think of it, also inspect the cable for signs of a kink even before you try to lube it with the graphite. You might see the problem.
If neither of these works, but a slightly shorter cable and install in its place, sans the tight S bend. Cheap & easy.
Remove the speedo cable and measure it end-to-end.
Then, get a new one that's 3" - 4" longer.
Bug City has them by different lengths.
With mine... I found it was jumpy because the cable wasn't threaded in all the way. it was TIGHT but it still bounced. I looked in the back and found that the knurled cable end was bottoming out on the bracket. I opened up the hole in the bracket a bit so the knurled end would slip through the hole in the backstrap / bracket. Once I did that I could tighten down the cable end a bunch more.
After I did that? No more jumping. 
You can bench test them simply with a speedo cable and a drill. See if it bounces on the bench with a straight cable. We've had a few that were something internal, but 99% of the time it is related to a dry cable or the bend in the cable as stated above.
I've had the same problem 2-3 times. Thanks everyone!
ummm I'm new here.........
Had a similar issue fixed it by doing all that was suggested ..
My original VDO Speedo now works fine and has no problem reaching 100mph
Don't believe me?!!!
Ask Anand............... as I chased his happy a$$ down the freeway in his F430 "oh us CRAZY Californians!!"
CRAZY!! look at that!! 3600 rpm almost 90 mph!! and more power there!!
Tebs
Poor Anand, he could not get out of third gear waiting for you.
Stop hitting bumps. Need a beer after that.
