I have been trying to figure out why my Rally Pac Tach is reading high compared to actual RPM and still have not clue. Without having removed the tach I simply did some reading with a timing light and compared to what was being displayed on the tach itself. The below is the chart of Tach versus Actual RPM.

Just a little background, this is a manual 69 442 with 455 and HEI distributor. Any guidance would be great and help me immensely.

Actual RPM --- TACH RPM
760 (idle) --- 1000
1500 --- 2000
2000 ---2800

Regards
Sean

 

Mine is the same. 68 Rallye pac tach. No HEI. I guess the up-side is my 70 455 red lines at about 6500 RPM's. :/

 

I don't know anything about these tachs, but the 25% descrepancy is the diff between 6 and 8 cyls.

 

Every car I've had over the last 30 years w the Pac (factory or added by me) was off to some degree similar to yours. NEVER seen one accurate

 

It's been 20 years since I had mine out, but I think I remember an adjustment screw in the back.

 

Frank,

Correct, there is a adjustment screw on the back (behind the foil tape). I will try and pull the gauge and see if I can get it close to actual.

Sean

 

My 70 was off took it out and hooked up another tack, and run the two together, and adjusted the back screw. A little move moves it a lot, so may take a couple try's. Also check it at low and high speed.

 

That adjustment is very sensitive. I turned mine back and forth across the full range (gently) to try to clean the control. It seemed to help.

 

I completely agree. My cars and my customers cars have all been off. I guess the factory didn't worry about being accurate as long as it looked good. ~BOB

 

So I got the tach calibrated this weekend by simply connecting a digital timing light with RPM display and making it visible while I was behind the wheel. I then reached up behind the dash to access the rear of the tach where there is the small adjuster screw (covered by foil tape). The best way I found of adjusting the screw was to us a phillips socket insert and just insert into the screw hole until you feel the screw being engaged. I then gave the car some gas until about 2500 RPM on the timing light and held it while I tried to match by adjusting the tach screw. This is very sensitive so it might take a few tries to get it right. Only about 30mins of work and it now appears that the tach is as good as new.

Sean

 

Does anyone know if it is the same for a '67? The tack in my '67 442 is reading about 20% high. I'd like to get it dialed in.

 

On a tach-related note; has anyone ever seen an extra fuse taped to the bottom from the factory?? I swear I had to be the first to remove this tach from my 72 U code Supreme which came with the gauges from the factory and that I have owned since '94.