Kensmith

My tachometer reads about 200 RPM less than it should at highway speeds. I have checked the speedometer, calibrated the transmission, rear end, and tire diameter and everything points to the tach. Is there an easy way to calibrate or should I leave it alone as 200 RPM is no big deal. At idle it's dead on, only when I get up to highway speeds is it off. Thanks

GCD1962

There is an adjustment on the tach head, but ithe unit has to come out of the car. You could probably calibrate it yourself if you also install a temporary electric tach. The adjustment on the tach is quite simple.

C2Driver

Ken, I'm just curious - How do you know that it's out 200 RPM's at highway speeds? Are you running a separate electronic tach to check it?

- Pat

Kensmith

No, I have a couple different rear ends I have tried along with gear calculators and no matter what rear, it's 200 RPM lower. Only at higher speeds as I have checked it at idle with a dwell tach.

Kensmith

I'm interested in this, any way it can be reached from under the dash?

C2Driver

Ken, if the tach is spot-on at idle, it's entirely possible that it's also spot-on at highway speeds aaaannnnd the gear calculators are inaccurate. I've used a few of the online gear calculators and they all pretty much look alike. If the original developer made a small error in programming, I suspect that many clones/copies will have the same error. Before doing anything, I'd suggest doing a quick hookup of an electronic tach, to compare to your tach. If you don't happen to have an old tach laying around, I suspect your mechanic buddy might.

My 2 cents.

- Pat

GCD1962

Don't really know on a C-2, I have a '62. If it is similar you need to physically get to the tach head within the unit. I doubt you could get to the adjustment with it still in the car

Godholio

Nope, you'll have to yank the whole instrument cluster and remove the tach.

ejboyd5

A very small mismeasurement in tire diameter can easily account for a 200 rpm perceived variation at speed. It is much easier to try several electronic tachs via temporary mountings to check the car's unit than it is to swap rear axle ratios.

redred65cpe

That's a very good point. The spec'd or measured height of a standing tire is not going to be the same when the tire is spinning (rotational speed and temp will cause variances).

larry

JohnZ

Tach calibration is the same as speedo calibration - it's done by gaussing/degaussing the magnet cup with specialized equipment (on a bench), not with any mechanical adjustment.

GCD1962

John - On the C-1 tachs I remember being able to turn the head one way or another (but don't trust my memory!) to adjust the reading? or is that just to zero it out?

bweaver999

Why not verify with the dwell tach? 3 or 4K RPM should be the same in the garage as driving, set the dwell tach so you can see it with the cluster tach and hit the gas. That and I would not expect 40 year old technology to be spot on, the cops used to give you 10MPH grace for speedometer error.

Chuck Gongloff

John is 100% correct as usual. My NCRS Chapter had a tech session a number of years ago at Corvette Specialties of Maryland.

Brian Tilles, the owner, and one of our Chapter members, demonstrated the gauss/degauss machine he has. Actually calibrated at few tachs and speedos for us as a demo.

You spin the tach (or speedo) at a dead on known RPM, then "zap" the magnetic cup either imparting or removing magnetism from it.

THAT'S the way it's done. NO external adjustments at all.

Chuck

Kensmith

So it must be removed to calibrate then. Thanks John. I will just calibrate my brain to +200 RPM while cruising down the road!

gldnkngt

The adjustment of the hair spring on the tach head has to be done in conjunction with the proper gaussing/de-gaussing. You can tweak the spring to read what you want at highway speeds, but that may throw it out of whack at lower speeds.(or higher speeds.)

0Brian@CSofMD.com

Absolutely correct! There is a small "arm" called a regulator that adjusts hair spring tension to "fine tune" the unit after the proper magnetizing, and demagnetizing procedure is followed. The unit must come out of the cluster to be serviced. Your tach is probably closer than you think, I would leave it alone. Another thing to think about, do you know that the speedo is "on the money" ? It may be off just a few mph and is compounding the problem.

GCD1962

That's what I must have been thinking of "the small arm". I needed to adjust that when I installed a nos unit in the tach. Thanks for the clarification. In the "fine tune" how much of an adjustment is there?

Kensmith

Yep, speedo is correct, tach is off at higher speeds. 200 lower at 65 mph, 300 lower at 75 mph. So it is progressive.

0Willcox Corvette

This was not an instrument of accuracy even when new! The tachometer calibration allowable variance is +/- 100 rpms so you are not far from that with what you have.

Willcox Inc.