wumpus wrote:I strongly suspect the computer counts the number of times it sends pulses to the sparkplugs and and moves a motor to indicate RPM. I'm pretty sure I've seen dashboard mountable tachs that simply read the ODB(II) data and displayed it in LED format. I also recall somebody claiming that during a sportbike scandal where a certain engine was supposed to rev to umpty-up warp rpm, the engineers who calibrated it knew they where lying since they were moving a stepper motor from data in the computer. YMMV.
I'm pretty sure whatever method is used is exactly the same as the speedometer. One reads before the transmission, one reads after.
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