Tach reads high - Causes

[72 Challenger]

Hi Everyone!

It's been a while but that's because I put my car away for the winter! 

Full disclosure, my tach issues are with a 1970 Duster, but this could happen to any car.

My factory rallye dash has a tach that reads high by almost 2x. (1,000 RPM = 2,000 RPM) What would cause this? It was my understanding that the tach was just based on how many times your coil is firing, seems like a simple enough electrical read out. When I use an old school engine analyzer I can get an accurate read by hooking it up to the positive and negative on the coil.

I'd like to know what my engine is actually doing because it was built for a redline of somewhere around 5,500/6,000 but obviously when you multiply that by 2 the needle is well after the numbers on the tach.   

Any ideas of where to start this search for possible solutions? Ideally the answer is not Dash rebuild because it's fully original and easily the nicest Duster gauge cluster I have ever seen.

[Bullitt-]

 If you have an aftermarket electronic ignition with multi spark, as many do, it will mess with a factory tach.

[Mr Lee]

Do you have electronic ignition?  I believe once that is added to a car it can mess up the operation of the tach. 
Real Time Engineering makes a circuit board that replaces the original in the back of the tach.  It has tiny little screws in the back that let you calibrate it.

Or maybe it just needs to be gone through.  I'm sure someone here can recommend for that

[72 Challenger]

Good points, the car was not an electronic ignition car from the factory, but when the 416 was put in electronic ignition was added. It looks to be a stock looking mopar orange box.

There is no aftermarket box like an MSD.

[JH27N0B]

When I got my T/A back together, I noticed the tach reads high also. Maybe as much as 2X.
Before disassembling the for restoration it seemed accurate, albeit sluggish.
It had an Accel distributor with no vacuum advance from the 70s on it before and Accel super coil.  Otherwise stock.  When restored I went bone stock original distributor, coil etc.
A friend said there is a calibration involved with the tach and suggested I'd need to tear the dash apart to get the tach out to recalibrate it, and that when I'd had the gages restored is when things would have got out of whack.
I don't know enough about how these work to know if he's right, but would like to fix it. 

[72 Challenger]

Who are we using to rebuild guages now? perhaps I could call them and ask if they think it's a gauges need restoration issue or because of the swap to electronic ignition.

[blown motor]

There's a place in Harrowsmith called Premium Dash Decals. I sent my dash there to get restored and have the tach recalibrated. It came back and the tach was still the same. I sent it back and it's better now but still not near accurate. If you send your dash or tach out don't send it to them even though that would be handy for you.

[72 Challenger]

There's a place in Harrowsmith called Premium Dash Decals. I sent my dash there to get restored and have the tach recalibrated. It came back and the tach was still the same. I sent it back and it's better now but still not near accurate. If you send your dash or tach out don't send it to them even though that would be handy for you.

Just looked it up and it's the same town I bought my first mopar in.

I wasn't aware this guy even existed.

[DeathProofCuda]

I'd like to know what my engine is actually doing because it was built for a redline of somewhere around 5,500/6,000 but obviously when you multiply that by 2 the needle is well after the numbers on the tach.   

If you want to know what your engine is actually doing you may not want to rely on a factory tach for that information.  The internals of the factory tachs are heavy and very slow to respond, so there is usually a lag between what the tach is reading vs. actual engine rpm under heavy acceleration, which is why many cars with factory tachs still have aftermarket tachs installed on the steering column.  If you don't want to go that route, I'm pretty sure that companies like Redline Gaugeworks can install modern tach internals in a factory tach housing.

[72 Challenger]

I am not to worried about how exact the tach is, close is good enough. I will contact redline and see what they say.