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.
I don't *think* this is entirely correct when you consider that a car's computer, in addition to running the car and displaying the necessary information, have to monitor the functions. If it was simply indicating how many times it was pulsing the plugs, it could get things off, especially in cases where there's a non-computer reason for things to be changing, such as downshifting a manual car without rev-matching. There's almost certainly still a way for the computer to have the information and ensuring things match.