The tachometer is one of the few things I just haven't gotten around too in my Civic EV, and all I need to do it wire the countershaft wire harness sensor to my Zilla!
DC motors have sweet spots where speed/current/cooling fan operations are the best. My Warp9 has a recommended top speed of 5000 RPM, but likes to be around 3000 RPMs for best performance. I think the best RPMs really depends on the car and gear ratios.
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