Got one of those inductive loop tachos from the seventies to fit on a Lotus twin cam which has coil ignition but it overeads the rpm by nearly twice.Think it may be for a 6 cylinder car.The dial ref. is rvi1003/00.Anyone confirm what it is.The inductive loop polarity and loop size,is it important,could that cause the overead.Should add the dizzy is fitted with aldon ignitor.

If it was for a 6cylinder it would under read, most like somebody has twiddled the calibration screw, only way I know to calibrate it with reasonable accuracy on car is to use a O-scope to measure the frequency of the LT voltage.