After my trip in the 1972 W-30 to Three Hills for the car show I got to thinking that at 65-70 mph cruise speed running with the pack to the show there was no way I was spinning such high RPM as the factory tac suggested. At 70 mph the factory tac read 4200 rpm. Not a chance I was revving that high....my ears are not that bad.

So today on a back road I drove 30/40/50/60/70 mph in 4th & wrote down the RPM indicated on the factory tac. Then I swapped the tac out two times to test my spare factory tacs. A third spare tac I found out is NFG.

So here is what I found using the factory installed Rallye Pac tac that is original to my car (speedo is bang on accurate verified by GPS)


30 mph = 2000 rpm observed but 1380 rpm calculated
40 mph = 2300 rpm observed but 1840 rpm calculated
50 mph = 2900 rpm observed but 2300 rpm calculated
60 mph = 3600 rpm observed but 2760 rpm calculated
70 mph = 4200 rpm observed but 3220 rpm calculated


This is with the Muncie 4 speed & factory 3.73 gears & a 28" tall rear tire.


Oldsmobile sure built themselves a warranty replacement insurance policy with these factory tacs. They are calibrated way off. The better of my two spares reads another 100-150 rpm higher across the tested range.


While I had the dash apart I tested my two spare gauge pods. Pleased to discover that they both work & read the same as the original. She is all put back together now with original pieces (of interest the date code on the gauges is June 14 1972 & the car was built on June 29).


Next time I drive her she is going to the redline and then some. No wonder she was still pulling hard at 6000 rpm!

 

I replied on FB but have you thought about trying to calibrate it? It kind of drives me nuts that I shift at 6500 rpm's on my factory tach on my BBO by feel. I know it has more but I'd like to know what the real rpm's are before I push it.

 

You can calculate what the theoretical RPM is based on your speed, tire height & gear ratio.


And how are you suggesting that the gauge be calibrated? Please explain.

 

Yes, With the screw on the back and a dwell meter set in rpm mode (I assume the 68-69 is the same as the 70-72). Why wouldn't you just make the tach read correctly?

 

So I just read the 68 FSM Section 15.


There's a screw that reads "tachometer calibration" shown on Figures 15-68 and 15-70 but there isn't any procedure for performing the calibration. Later on in the section it says "No not attempt to calibrate tachometer as circuit board is calibrated at the factory and requires no adjustment"


I've got to pull mine to see if there's a calibration screw there. I know years ago there was a post on ROP about calibrating the factory tachs because they were never right. It's long gone now.

 

It seems that this was discussed on this forum before. There is a a foil type sticker covering the screw access, and the screw is really sensitive, adjust it as you have described (allyolds68). When my '69 was running, it was reading 4300rpm @ 60mph with 3:42 gears, so I'm glad that the calibration may be an easy fix, if i ever get it going again, I'll give it a shot.

 

I was not aware of the process. If it is so easy & so common I would think that there would have been a sticky or a thread on this somewhere. I'll bet I am not the only one who would like to know how to do this nor even aware that the factory tac calibration is a joke.

 

Dave,


There have been a few threads on it:


https://classicoldsmobile.com/forums...-tach-off.html


You even replied in this one:


https://classicoldsmobile.com/forums...-too-high.html

 

That 1st link is a useful one. Thanks.


I'll have a look at one of my spare tacs & play with it. Don't want to mess up my original one or make it worse than it already is.


Strange that the FSM states clearly NOT to make adjustments or attempt to recalibrate the tac.

 

I think you hit the nail on the head. It was probably a cheap insurance policy against over-revving the engines.

 

I have the same isue with my Tic Toc tach. My question is there any difference
for calibration with HEI ignition.
As an old not very limber old fart, I only want to attempt calibration once.
Thanks for any advice.
Jerry