One of the tricks after taking ONE loop out --that is simply passing the wire through once--- is to take the back off and locate the adjustment pot(entiometer). Drill a hole in the back so that a small screw driver can be inserted to make tach adjustments "on the fly". I have a Crane unit installed and set it to read correctly at mid-range. Knowing the speed vis a vis tach readings --with a 4.22 rear end every 1000 rpm ~ 15 mph,--- I drove at 60 mph and set the tach to 4000 rpm. .. .. it may not be EXACT, but it's close enough for me...
Be sure to do this on a relatively empty, straight road. It can be a challenge to get the screw driver into the back of the tach at speed.