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I assume you're talking about stock Mazda gauge clusters?
The short answer?
Significantly.
I can only comment about stock FC tachs, since that's my experience.
Mazda has an "adjustment calibration screw" on the circuit board to compensate for such variations.
When we were messing with these things, we've seen these things regularly off by an average of 400 - 500 RPM's.
I've seen some off by almost 700 - 800 RPM's, but this was a rare case.
Readings were done at discrete 1,000 / 2,000 / 3,000 RPM increments.
Signals was generated with a custom built microcontroller.
Signals was monitored with a lab-grade o-scope.
-Ted
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