Yes...I believe the speedo cable binding could definitely cause that. In fact, it has always seemed to me the cable should be ignored and the unit should have been designed to mount directly to the speedometer (right where the cable would)...but if the adapter makes an annoying whine noise, I understand why the maker wants it outside the cab. At low rpms, the stepper motors driving these adapters may not have enough "oomph" to turn a cable and a mechanical speedo mechanism with any accuracy. In the electronic speedometer the needle attaches directly to the stepper motor shaft so the thing can be accurate and very smooth. Please take a look at what I'm doing on this...in the electronics section. You'll see the stepper motor controlling the speedo needle is right in the center.
