S13 HUD windshield & Tachometer reads fast.

[Woods Crasher]

Hello everyone. I have a 92 240sx with heads up display and i have to replace my windshield. The HUD windshield with the little embedded polarized tint in the windshield is over 1000 installed. So I am going to install a non hud windshield for about 70% off that. My question is; Does anyone know where I can get a film or tint that's reflective and polarized?(polarized to stop the light from the hud reflecting off the outer layer of glass causing ghosting)

Now with my tachometer. I've noticed a few days ago that my tachometer is reading fast. It shows my idle at around 1200 when the engine is idling 750ish. I gave it a good pull in second to maybe 6000 rpm(about 50 mph in 2nd) and the tach was reading 8000). What can be causing this? Is it the tach itself? maybe some electrical problem? Has anyone had this problem?