Installed aftermarket tach, RPM's seem way too high!?!

Alright, I just got a Proform 5" tach from Tolowhb, and I finished installing it earlier today. Now the tach in the cluster is still working, and I've noticed that the Proform tach is always around 400-500rpm higher than the stock tach. I know that the stock tach isn't the most accurate thing, but I'm fairly certain it shouldn't be that far off. What do you think could be causing the tach to be so far off?

if you want to know if its off. take the car to rev limiter and if the tach matchs the cars rev limiter than you know the tach is accurate. also check to see when your car hits rev limiter that the tach reads that rpm, the same time you hit rev limiter