I might just order another speedo to see if the unit is faulty. Thanks for your help.

My bike is already equipped with a BEP 3.0. As instructed on another post, I am using the Beemer's original speed sensor as well. Everything else works flawlessly because of the BEP. I'm not seeing much information about a KBSI on the forum. You asked about a specific model and whether I have a KBSI (as an alternative to the BEP?)—do you have knowledge regarding whether I should be using a KBSI instead of a BEP 3.0 or maybe in tandem?

The trials and tribulations of riders installing an Acewell on a Brick are widely documented on this site. The reason I asked about an interface with the Acewell is that is seems not able to produce a steady speedo signal without one if the OEM sensor is used. Did you buy a BEP3.0  or KBSI with your Acewell? Below is a quote from the Acewell site page for BMW K owners. Maybe you've read this info already. Let us know.

You do not need our inverter ACE-IVR03 in combination with the BEP 3.0! Important! You will face these problems without the BEP 3.0:

- The battery is no longer charged - The start button no longer works (only with the clutch pulled) - The signal for the idle indicator cannot be tapped from the wiring harness - The speed signal from the final drive cannot be used - The speed "jitters" because it is not suppressed and digitized - The reserve signal at 4L cannot be used - The automatic indicator reset is not working

The solution:

The BEP 3.0 therefore fulfils all essential functions such as closing the LIMA/battery charging circuit, resetting the indicators or a functioning start button. Simply plug it in and drive off. You don't need to use a soldering iron or cut any cables on the wiring harness to make the conversion. The entire electronics fit into a small 10.3cm x 6.2cm x 2.7cm plastic housing and weigh just 80 grams. It can therefore definitely be described as compact and lightweight.

CV080-503 is the model I believe

The speed has never remained consistent on my Acewell Speedo, which makes it incredibly difficult to calibrate. When I ride, the digital speed will fluctuate between a range of values anywhere from 2 to 50+.

The faster I ride, the more the speed fluctuates, sometimes down to half the actual speed (i.e., if actual speed is 80, displayed speed fluctuates all over the place between ~40-80).

Sometimes the speed goes completely flat to zero when I downshift and coast. This happens rarely. Probably something else unrelated. Tightened the terminal bolts recently and haven't noticed that happen since.

The speedo is plugged into the BMW speed sensor. The Acewell speed sensor is not being used.

As far as I can tell, all the other electrical work I've done on the bike is sound.

Think it's a faulty speedo, ground issue (doubtful), or something else?