With modern electronics they should both read the same especially as they are both getting their signal from the ECU.
The gauge would be the most likely to be incorrect, probably something on its circuit board. It's only basically a voltmeter after all.
You can buy a cheap digital laser tacho - $20 Ebay - to compare. They're surprisingly accurate, I found.
Other option is connect a USB cable to the Ecutalk and run a different software like OBD Scantech to see what it displays.
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