Tach Reads High

[B5 Six Pack]

Trying to figure out which way to go with the factory/ralley cluster tach in my 73 cuda. First some background: Originally a 340/4 speed car, it now has a 440/ 4 speed. The transmission is not original to the car. It has the factory installed 8 3/4 rear with 3.55 gears. The first issue was that the speedometer was reading 10 miles per hour faster than what the GPS speedo app on my smart phone showed. That made sense since I knew the trans was not original based on its stampings. Changing the speedo gear from a 32 tooth to a 35 tooth made the speedo right on at 30 mph and within 3 mph at 60mph.
The tach reads 3250 RPM at 60mph (still using the GPS). For that to be correct, I'd have to have a 4.22 ratio rear end. The thing that confuses me is that the tach is very smooth at all RPM's. It indicates 900 at idle, which seems pretty close just by listening to the engine at idle. I don't have a test tach that I can connect.
Sorry for getting so wordy. The question is: Has anyone seen a tach that operates perfectly smooth, but reads too high? The car still has Chrysler electronic ignition. I'm using an Accel brand coil. Thanks for sharing your knowledge/experiences.

[Rich G.]

My friends tach does the same thing as yours does. Mine seems pretty accurate but I had the gauges rebuilt. You might want to give a call to Auto Instruments or Redline gauge works and see what they say can be done.

[dodj]

The OE tachs are not known for being very accurate. At least not now.  They are 40+ years old. Reading high is not uncommon. There may be a pot inside for calibration? But I'm not sure.

[RUNCHARGER]

Most of the factory tachs I have had do what yours is doing. They work smoothly and are close at idle but are optimistic at higher revs.

[B5 Six Pack]

Thanks to all who posted. It's good to know that it's pretty much "the nature of the beast" at this point. Maybe it'll keep me from scattering it when shifting.

[nsmall]

Mine read fine at idle.  Ran 1000 too high at 2500 and ran 1800 too high at 4000.  I know as I hooked up a gauge.  Had Redline rebuild the gauge ($280)...ouch.  Works perfect now.  It wasnt jumping at all so I thought it was fine, but it was way off before the rebuild.