I found out that the gap needs to be filled with a rubber gasket. The HD parts lookup shows this as a strip and the link below is what I bought. The OEM stock number is cross-listed to this part.

J&P Part #: 560-790

 

Glad you got it sorted.

 

Update****

Well, finally got the parts. Gotta love the long order recieved/shipped/ recieved time during this ordeal.

The rubber ring that is suppose to isolate virbration does not fit well at all. If I were to remove the rubber collar that fits between the speedo and the botttom bracket of the console mount, it would work. Changed rubber speedo mounts and ensured everything was secure. I just want to lay my hands on someone elses speedo and see what the difference is. Till that happens everyhing is put back to together and it may vibrate. Until I find a solution. And for as far as I know maybe thats the way its suppose to be? The Deuce speedo was tignt didnt move around. I just wanted to update.

 

I quit using the little rubber mount grommets, they would wear out on my bike after a few thousand miles, and the speedometer would shake like crazy, this ruined the original unit, because it got loose and broke the trip meter **** off....

I mounted the speedometer with bolts, spacers in place of the grommets, and nuts.

The solid mounted speedometer has been fine for at least 25 years.