Ratio of alternator speed to crankshaft speed with ribbed V-belt

The relationship between the alternator in a V-belt generator and one with a ribbed belt is quite different. I bought a tachometer from a newer Golf because mine was broken. Now, it's showing a value that's far too low – I'd say around 20% based on my feeling.
Does anyone have the engine speed data readily available, or the percentage difference between the two versions? Do you think it's possible to compensate for this by adjusting the potentiometer? I was considering resoldering the circuit boards, but the new one is a combined board with everything on it.

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