your tach is no relevance to how many rpms your engine is turning. The RPM on the tach is only a signal from the ecu, somtiems weaker and older clusters tend to be off, its not -+500, its a % off.
My stock Ek9 ECU turns to 8600 on the dot, my cluster reads 9000ish
Take it to a dyno to really tell how high its turning.
You can plug am aftermarket tach on the distributor cap and you will get a exact rpm... cause it counts the impulsions send. But try to keep your wire as short as possible else, you get kind of a delay