61 Corvette 6500 redline tach

[David S.]

I purchased an original rebuilt 61/62 corvette 6500 redline tach to replace my broken one. The car came with a 6500 tach, so this matches the original one from the car. Anyway I replaced my speedometer and tach with rebuilt items. The speedometer is not working yet, but the tach is working and is connected to my generator in my 61 corvette as it came from the factory. The person who rebuilt it said it was calibrated for my generator driven tach. The car retains either a 4.11 or 4.56 rear-end, unable to determine which one without taking apart. My question is on the calibration of the tach, which seems to read high, almost matches the speed of the car when driving the car up to 45 miles per hour. Does anyone have a 61 corvette with a generator driven tack and can tell me if this is correct? I was told the tach can be calibrated for a distributer or a generator; hopefully I have the correct one for my car.
Thanks,
David

[Tom B.]

Re: 61 Corvette 6500 redline tach

If your pulleys are correct and the tach is fast or slow then it isn't calibrated correctly. You can compare it with a dwell tach or temporary hook up an aftermarket electric tach.

Nothing to take apart to check your rear end gear ratio. Jack up the rear end with the car in neutral. Turn the wheel one complete turn. If it's a 4.11 the drive shaft will turn just a little over 4 revolutions for one turn of the wheel. if it's a 4.56 it will turn just over 4 1/2 revolutions for one rev of the wheel.

Tom

[David S.]

Re: 61 Corvette 6500 redline tach

Thanks Tom I will try that....

[Bruce B.]

Re: 61 Corvette 6500 redline tach

A generator driven tach and a distributor driven tach are not interchangeable.
The distributor driven tach is calibrated to run at 1/2 the speed of the generator driven tach.
Therefore the tach can not be calibrated to work in either situation.
Someone correct me if I am wrong.

[Tom B.]

Re: 61 Corvette 6500 redline tach

It's magnets spinning inside a drum. The strength of the magnets determines the pull on the drum controlling the swing of the needle. I can't say if there are different size magnets for different tach units or not but if they are all the same you could theoretically calibrate it to any ratio desired. I don't think that they change out the magnets but alter their strength instead. Then it's driven on a test cable run at a known speed to determine if the needle is accurate.

Tom

[Christopher R.]

Re: 61 Corvette 6500 redline tach

Dave, are you saying that when you drive 45 mph, the tach is reading 4500 rpms? If so, that's wrong. With my 3.70 rear end, the speed is about double the tach. That is, at 70 mph, the tach reads 3500 rpms.

[David S.]

Re: 61 Corvette 6500 redline tach

I did verify my rear-end gears and they are 4.11. Looks like from all the information provided, the tach is calibrated wrong.

[Tom B.]

Re: 61 Corvette 6500 redline tach

That doesn't surprise me. I had mine repaired and calibrated at a local speedo shop in 1971 and they set it several hundred rpm too fast. One of these days I'll take care of that.

Tom

[David S.]

Re: 61 Corvette 6500 redline tach

Wonder if I am better off cutting my losses and purchase a new replacement from Corvette Central for $369.00. I assume they are foreign made, but might work better than the one I have...thanks for your help.

[John H.]

Re: 61 Corvette 6500 redline tach

Dave -

Instrument rebuilders (like Corvette Specialties of Maryland, D&M in South Carolina, etc.) can recalibrate your tach for use with a generator drive (the one you have is calibrated for a distributor drive, which is why it reads 4500 rpm at 45 mph). Lots cheaper than buying another one.

[David S.]

Re: 61 Corvette 6500 redline tach

Thanks John, I will give them a call tomorrow on the calibration.

[Tom P.]

Re: 61 Corvette 6500 redline tach

ABSOLUTELY, either a mechanical tach or a mechanical speedo can be calibrated. I do not do calibrations, nor do I have the equipment to do so. But a GOOD technician at a speedo shop can do it. It requires partial disassembly of the speedo (or tach) case to gain access to the head. Then it is put in a machine and spun up (to the desired rpm) and an "adjustment" is made to "something" inside.
How do I know?
'Cause I watched the guy at my local speedo shop do it, that's how I know.
My 51 Chevy has a matching tach and speedo. The tach is made from a second speedometer which the speedo shop re-calibrated into a tach and it is driven by a Corvette tach drive dist. Works great!
When I took it in to the speedo shop, I just told the guy that I needed the "tach" to read twice the cable speed. No problem for him to re-calibrate it.