Talked to HD directly. I asked specific to adding a 31759-04A to a 2006 XL1200C. No speedo issues.

The only way a speedometer error should occur would be if the wrong model were used.
Example:
883 module on a 1200 and vice versa.
1200 module on a 883(1200 conversion) - final drive gears did not change so still need to use the 883 module in this setup.

I've got them on speed dial though. Just in case I'm proven wrong down the road.

 

Let us know how it works out. Make sure you mention that the trannys in the 05-06's are different, and ask where the speed sensor is at. If it's in the tranny, then the potential's there since the gears are cut differently. Ask them if they'll allow you to return it if it doesn't work.
Thinking out loud: Also, since the trannys are the same between the 883 and 1200, then why would it matter if the 'wrong' module is used? Maybe we just don't know where the speed sensor's actually located. (?)

 

On my 2005 1200C the speed sensor is mounted on the top of the crankcase behind the starter.

The Clymer Manual states:

"The VSS monitors gear tooth movement on fourth gear and sends the data to the ICM".

 

Hey, I hope it works for you, but all I know, is when I put in a 31784-04A for 04 and later XL 883 models (as it says on the package), my speedo was reading 10 mph fast........... Put the stock module back in, and it was fine..........

 

But, this is good, it reminded me that I want to put a spare module in my ditty bag, for "just in case"

 

Well that makes sence. Does anyone know if a stock 05 ignition module is the exact same part number as a 06 stock ignition module?

 

My stock 06, 883 module has numbers: 32478-04B 05313

 

HD came out with an ignition in mid november that works with the 2006. It is the racing ignition #32471-06 and the tach. and speedo work with it. In September I had the dealer install the street legal ignition and it ran great but my speedometer was off. When the new ignition became available the dealer switched it out for free afterI threatened to complain. I have a 2006 1200R.