Boilerz25

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

allyolds68

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. :/

Indy_68_S

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

pizza442

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

frankr442

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

Boilerz25

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

442Harv

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.

frankr442

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.

Carshinebob

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

Boilerz25

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