No inverter.

Hud will read 0 and HPT will never read zero unless the engine is off, goes as low as 1 kph.

I can get the HUD to read a couple KPH by revving the engine in park. That would probably be a key detail which would suggest there is not much of an issue with the HPT so much as the data itself.

I have removed and cleaned up the chassis ground, have not touched any other grounds.