Tachometer Issue

[boyzone]

Hi there, recently I just got myself an old Pivot tachometer, and I installed in by connecting my RPM signal wire. The meter has accurate idle reading, but when I rev up my engine or running my car, the tachometer no longer accurate,

Let's say my stock dashboard gives 2000RPM, the Pivot is giving more than 2000RPM which is faster RPM reading.

So may i know how to solve the problem of this? Oh ya, I've uploaded some of the images of my tahcomter, can someone let me know what model is this Pivot tachomter?

Many thanks :D