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Gauges are an unpredictable load. They have an inductive component, but it's often not enough to filter the PWM signal to an average value.
When testing with a 25KHz PWM signal and MOSFETs with negligible on resistance, we found that most Miata gauges required 50-95% pulse width to operate. Each gauge type reacted differently, with some generating enough "kick" to require a freewheel diode.
The point is that you aren't going to be able to guess at the appropriate pulse width for a specific gauge reading. You really have to sit down and try it out, building a calibration table.
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