Speedometer slow after new locking front diff installed.

Syncronoid

'90 Syncro Westy. 16" wheels.

I just had a rebuilt locking front diff installed (rebuilt with 4.86 R&P, formerly a 5.83 R&P). This locking front diff was imported by Autostadt in Sacramento and rebuilt by GTA.
I started out from the shop today after the install and noticed the speedometer was slow and it dawned on me they hadn't swapped the speedo sensor from my old, stock, US, non-locking diff. So, I went back and they swapped the speedo sensor. However, while putting it in, we noticed that the speedo sensor from my old non-locking diff was larger (more teeth) than the speedo sensor that came out of the new european locker (19 teeth). We put it in anyway and now my speedometer is still slow (maybe even slower), about 20% slow. I estimated using the RPM's. I usually would be at 65mph at 3500-3600 RPM's, where as now I am only reading 55mph at the same RPM's. This leads me to believe that the gear inside the front locking diff used to spin the speedo sensor must be smaller than the one in my old non-locker. Does anyone have experience with these???

Is there something inside this imported locking diff that causes the speedometer to show slower while using both the European speedo sensor AND the the speedo sensor from my old non-locker?????
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'90 Syncro Westy w/GW2.5L

syncrodoka · Posted: Fri May 29, 2015 10:35 pm Post subject:

There is the speedometer drive gear in the front diff that turns the speedometer drive that should have been swapped for the 4.86 version when the build was done. Swapping the speedometer drive with a mismatched drive gear will strip out the plastic teeth of the speedometer drive.

Example of the drive gear and the speedo drive.

Image may have been reduced in size. Click image to view fullscreen.

Syncrozilla · Posted: Sat May 30, 2015 7:11 am Post subject:

Here's a thread with morre info on the different speedo drive gears http://www.thesamba.com/vw/forum/viewtopic.php?t=597614
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Syncronoid · Posted: Sat May 30, 2015 8:44 am Post subject:

syncrodoka · Posted: Sat May 30, 2015 9:19 am Post subject:

The difference between a 4.86 and a 5.83 gear ratios is almost exactly 20%. Shocked

Since you started and ended up with a 4.86 with no tire size change and there speed reading is off the same as the gearing ratios it has to be that drive gear. I would talk to the builder about the issue.

Syncronoid · Posted: Sat May 30, 2015 12:12 pm Post subject:

syncrodoka · Posted: Sat May 30, 2015 1:03 pm Post subject:

The speedometer drive gears in the diff have either 7, 8 or 9 teeth, what do you have? Using a 8/19 on a 7 tooth speedo drive gear will strip it.

Syncronoid · Posted: Sat May 30, 2015 5:52 pm Post subject:

syncrodoka · Posted: Sat May 30, 2015 8:31 pm Post subject:

That is the plastic toothed gear assembly that the speedometer cable attaches to not the gear in the diff.

Syncronoid · Posted: Sat May 30, 2015 9:38 pm Post subject:


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