This problem had me going in circles for the last week, so let's see if we can figure this out:

My tachometer readings are litterally "half" of what it should be, meaning that at idle, 750-800 rpm, it reads about 400. At about 2500 RPM, the tach reads at about 1300 RPM.

I have a Microtech LT8, so I was using the tach readings from that, which are indeed correct.

What could be the problem? I have not tampered with the stock tach wire that goes to the trailings. The car is an 87 TII that drives and runs beautifully.

 

maybe you should tamper with the tach wire that goes to the trailing coils. check them for continuity/replace them

 

You may have lost the connection on a resistor or capacitor on the circuit board for the tach. The tachs can be recallibrated for a v6 (now they are calibrated for a four cylinder.) (Yours appears to be calibrated for a V8.)

 

Did it happen all of a sudden, or after the microtech install?

 

well, I bought the car as a shell, and restored it, so yes, after the microtech install.

The car runs beautiful, absolutely no problems with the engine and everything else, but it's this problem that is bothering me.

 

Mine did that as well after the Haltech install. I eventually just unseated the tach because I was getting annoyed with people asking me questions about it, haha.

Perhaps you should purchase a subtle aftermarket tach that displays the reading from the Microtech? It'd be more accurate anyway.

 

Replace the numbers on your tachometer with numbers that are twice the old ones. :P