I dont think the speedometer head would vary from US to EURO. I think the drive circuitry would change based on diff ratio (at least thats how I would have done it) and since you said it was ok before the change we can assume that the head is Ok and correct.

There are only three or four solder points so thats a low probability of fouling up.

I would replace the capacitors and see what that does. Perhaps the new gears added additional drag on the motor causing it to require more torque/current from the capacitors and if they were marginal to begin with the additional load might make them slow down..

Maybe. Just speculating.

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