[67gtospud]

When I restored my 67 GTO I bought a new repo tach. I think it was from the parts place but I can’t remember for sure. I’m pretty sure it’s not very accurate but don’t know if there’s a way to adjust it or calibrate it. At 60mph the tach is reading 2900 rpm and I believe it should be around 2500. I’ve verified my speedometer is within 1mph based on gps speed.

The 2500 rpm figure is off my 1:1 trans gearing, 27” tire, and 3.36 rear gears.


I have a sunpro tach for one of my other projects I. Going to hook up this week and compare them side-by-side but I’m definitely thinking the tach in my car is off. If I can’t fix or adjust it I’ll probably upgrade to a rally gauge cluster next winter

[TCSGTO]

I have a Pertronix Digital HP box and it has a tach calibration feature. You tell it what rpm and it sends a steady signal to the tach. My factory tach is 10% fast at all speeds. Most of them are, probably to keep people from blowing engines under warranty.

The Digital HP also sends a signal to the tach to flash to the set rev limit when you turn the key on to start.

[Formulajones]

Hmmm, Are you figuring in converter slippage with your formula?

I'd compare it to another known good tach before I made any decisions, or simply hook up an old dwell meter that has a tach function. My old Dwell meter has an excellent analog gauge movement on it that is very accurate.

I installed a hood tach on dad's GTO a couple years ago, I think it was also from the Parts Place. I always wondered how accurate it was. it seemed to be pretty close from my calculations. Well this weekend he wired up an Autometer shift light that has a digital tach readout in 10 rpm increments on it. Had them both running simultaneously and found that hood tach is exactly correct. It's a little slow to move up and down when you rev the engine, but at steady RPM it dead nuts.

Just an FYI, some of the aftermarket circuit boards used in the OEM repop tachs today have a calibration screw on them. If it has one, it's a little brass screw on the end of the circuit board. have to completely disassemble the tach to get to it. It's there for the technicians to calibrate the tach before it's sent out and not really there for us to tinker with, but it's an option if need be.

[Formulajones]

Here's a place that sells new circuit boards. If you don't see a specific application listed there you may have to call. Some of these have an adjustable calibration.
https://www.cajuntachshop.com/shop?page=3

[389]

I have a hidden shift light... The cable driven tach's seemed to be the most accurate..

[lust4speed]

Just to add another variable in to the mix we have had two different GPS programs running on separate cell phones and they themselves were a couple MPH different when we were trying to verify the car's speedometer. Makes it tough to check things when your checking devices won't agree.

[Formulajones]

Originally Posted by lust4speed

Just to add another variable in to the mix we have had two different GPS programs running on separate cell phones and they themselves were a couple MPH different when we were trying to verify the car's speedometer. Makes it tough to check things when your checking devices won't agree.

Can't even trust those digital speed limit signs either. They have them on the highway, a few of them about a mile apart from each other. I was going 65 and the first one said 66 so I was good with that. The next one said 74 mph, I was still going 65 lol. I was the only car on the highway.

[67gtospud]

Originally Posted by Formulajones

Hmmm, Are you figuring in converter slippage with your formula?

I'd compare it to another known good tach before I made any decisions, or simply hook up an old dwell meter that has a tach function. My old Dwell meter has an excellent analog gauge movement on it that is very accurate.

I installed a hood tach on dad's GTO a couple years ago, I think it was also from the Parts Place. I always wondered how accurate it was. it seemed to be pretty close from my calculations. Well this weekend he wired up an Autometer shift light that has a digital tach readout in 10 rpm increments on it. Had them both running simultaneously and found that hood tach is exactly correct. It's a little slow to move up and down when you rev the engine, but at steady RPM it dead nuts.

Just an FYI, some of the aftermarket circuit boards used in the OEM repop tachs today have a calibration screw on them. If it has one, it's a little brass screw on the end of the circuit board. have to completely disassemble the tach to get to it. It's there for the technicians to calibrate the tach before it's sent out and not really there for us to tinker with, but it's an option if need be.

I didn’t account for any converter slippage since my car is a 4 speed manual. I don’t have a dwell meter but I will compare it to another tach and see what the results are.

[70GS455]

To answer the question, yes they can be calibrated and adjusted. Just need someone with a signal generator and frequency counter...like myself

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