The RPM reading was off. I compared the reading to my APEXI VAFC and based on the gearing calculation versus the speedometer reading.
I did a searching on the internet on how to calibrate and all the information I got, was to remove the needle, set the car at idling & plug it back, which I didn't want to do as this method might damage the meter.
Thinking how did the manufacturer did their calibration, I removed the meters from the clusters and found the answers!
If u take a look at the top of the RPM meter, u will see 2 potentiometer aka pot (variable resistor). This is where you will do the calibration or adjustments.
Since it cant be adjusted once it installed back into the clusters/casing. I used crocodile clips to connect to the back of the clusters to get the signals. (where the screws are holding the meter in place are the contact point)
Next step is to start the engine and adjust the resistors by comparing to my VAFC readings. Frankly speaking, I do not know the reason for the two pots. Anyway, with the meter facing you, turning the pots anti-clockwise will lower the rpm reading. If u max out one of the pot, and the rpm is still too high, just turn the other pot also.
Instead of using VAFC, u can try to borrow a RPM meter that is known to be accurate and use it as a reference.
Hope this will help most of you who have inaccurate rpm readings.