The speed/rev relationship doesn't change.. the Actual ground speed does..
So, if one were check speed accuracy , tach rpm can be used .
Knowing ratio and tire size tells one what RPM will produce
Actual speed..
I drive 2 of my Vintage cars and monitor speed via tack [ per 1000 rpm] as one has had diff. gear change and the other is Km only...just by habit..
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