What rear end do I likely have.

[gbullman]

When I bought my 66 about 2 1/2 years ago it had undersized tires on it so speedometer read a bit fast. Last fall I put 215/60 R14 tires on and now the speedometer reads very close (within 2 MPH at 70 MPH according to a GPS speedo app on my phone). What has surprised me a bit was engine speeds are higher than I was expecting. At 70 MPH I’m turning very close to 4000 RPM, I was expecting more like 3500 assuming I have 3.55 gears in the rear.

So is it likely I have 3.89 gears in the rear? How common of an option would that be for a 140 HP Corsa Convertible?

Under most of my driving conditions RPMs are not an issue but occasionally I do some longer trips that involve decent length stretches of highway. Hopefully turning up a steady 4000 for an hour or so is not an issue for what I’m quite sure is the original, unmolested engine.

Thanks in advance,

Gary.

[66vairguy]

When I bought my 66 about 2 1/2 years ago it had undersized tires on it so speedometer read a bit fast. Last fall I put 215/60 R14 tires on and now the speedometer reads very close (within 2 MPH at 70 MPH according to a GPS speedo app on my phone). What has surprised me a bit was engine speeds are higher than I was expecting. At 70 MPH I’m turning very close to 4000 RPM, I was expecting more like 3500 assuming I have 3.55 gears in the rear.

So is it likely I have 3.89 gears in the rear? How common of an option would that be for a 140 HP Corsa Convertible?

Under most of my driving conditions RPMs are not an issue but occasionally I do some longer trips that involve decent length stretches of highway. Hopefully turning up a steady 4000 for an hour or so is not an issue for what I’m quite sure is the original, unmolested engine.

Thanks in advance,

Gary.

Your tire size is about as close to stock 7.00x13 revs per mile as you can get. At 70 the speedometer should read about 72MPH - normal for the time. Although speedometers can read a bit slow after five decades.

My gearset table says at 70MPH a 3.55 should be at nearly 3,500 RPM. A 3.27 should read about 3,250 RPM.

Non standard ring gears in LM's are uncommon.

My first thought is that the tachometer is not reading accurately. There is a gain access hole, but it is nearly impossible to use with the tachometer in the dashboard. It ONLY adjusts the gain so you need to confirm the tachometer reads fast by same percentage over full scale. I've repaired a few of Corvair tachometers and stopped doing them. The electronics is not expensive to fix, but balancing the needle (painting them throws them off a LOT) took me on average FIVE hours to get the accuraccy to +/- 500 RPM. The other issue is the meters are failing and THERE IS NO FIX. I had one I fixed and a month later the meter failed. Not my fault, but to keep the person happy I bought a used tachometer to get a good meter and fixed it at no charge. That is the last one I did!!!

[gbullman]

66vairguy,

I can totally live with the Tach reading high, in fact with the way I like to get into it that is probably a good thing. I’ll do some comparing with the Tach in my Tach / Dwell Meter just for another point of info.

Engine certainly is not complaining ant any RPM and seems to like the revs so not too worried.

One point I forgot to mention is I have an XR700 ignition installed and wonder if that can affect the Tach reading.

[66vairguy]

66vairguy,

I can totally live with the Tach reading high, in fact with the way I like to get into it that is probably a good thing. I’ll do some comparing with the Tach in my Tach / Dwell Meter just for another point of info.

Engine certainly is not complaining ant any RPM and seems to like the revs so not too worried.

One point I forgot to mention is I have an XR700 ignition installed and wonder if that can affect the Tach reading.

If the XR700 is installed properly (stock ballast and coil) it will not effect the stock tachometer accuracy . When I rebuilt the tachometers I tested for voltage and dwell variation and the Corvair tachometer is not affected by normal variations (it would have to be so far off the engine would not run). For the time period the tachometer design was top notch. A good design.

[JohnDB]

I suspect you have a 3.55 diff and that the tach is not perfectly accurate. When I balanced my carbs I had my tach/dwell meter hooked up and it reads at least 200 rpm lower than the tach in the dash. I'm running 195/60 15 tires and they look to be the same overall diameter as your tires. I'm mostly around 3750 rpm at 70, but if the tach is hot sometimes it seems to read a bit higher. In reality, it's probably more like 3500 rpm.

[gbullman]

Thank you to both of you, I agree I likely have a 3.55 rear, I’ll look for some markings before I put the car back on the ground later today.

If I’m remembering correctly with 3.55 rear and correct sized tires the relation ship between speed and RPM is sort of half; 40 MPH is about 2000 RPM, 60 MPH is about 3000 RPM, 70 MPH is about 3500 RPM. What I’m observing is that relationship adds more RPM at higher speed, so at 40 MPH I’m pretty close, 60 is more like 32 or 3300 so the Tach seems to be reading higher than actual as the RPMs increase. That actually would be good to me because I do some spirited launches and do flirt with the redline from time to time.

66vairguy, I installed the XR700, replacing the points the car came with. I left the resistor wire in place as well as the original coil so I would expect no issues with the voltage or overall resistance of my ignition setup.

Like I’ve said the Tach reading a little high is OK with me, it can only help keeping me out of trouble.

[gbullman]

Oh well, took a good look where markings are supposed to be (right side, close to transmission according to Clark’s) and see nothing that suggests a stamping. I also see from Clark’s that they don’t list a 3.89 for 65 to 69 so mine is almost certainly a 3.55.

Done worrying about this, now hoping the oil pan and valve cover gaskets I replaced yesterday don’t leak. The valve cover I did back in June is bone dry as is the lower shroud on that side. Hoping for same success on these.

[gbullman]

FYI, there seems to be some temperature sensitivity to the Tach. Today 70 MPH was about 3700 RPM, hotter days it was definitely 4000 under similar conditions. Best I can tell the speedometer is pretty solid so thinking the Tach is what is varying.

[66vairguy]

FYI, there seems to be some temperature sensitivity to the Tach. Today 70 MPH was about 3700 RPM, hotter days it was definitely 4000 under similar conditions. Best I can tell the speedometer is pretty solid so thinking the Tach is what is varying.

When I repaired these units I replaced ALL the capacitors and resistors. They had about a 10 year design life span and are well past that. The quality of these parts was certainly good, but today there are better alternatives. Heat can affect the transistors. As I mentioned before the biggest issue is the meters fail and cannot be fixed.

I have designed a modern replacement using the original case, dial, needle to look just like the original. I won't be done with the real world durability testing until later this year. I don't thing there is much demand for replacement Corsa tachometers so this is just a hobby thing for folks who want there tachometer to work.

[gbullman]

When I repaired these units I replaced ALL the capacitors and resistors. They had about a 10 year design life span and are well past that. The quality of these parts was certainly good, but today there are better alternatives. Heat can affect the transistors. As I mentioned before the biggest issue is the meters fail and cannot be fixed.

I have designed a modern replacement using the original case, dial, needle to look just like the original. I won't be done with the real world durability testing until later this year. I don't thing there is much demand for replacement Corsa tachometers so this is just a hobby thing for folks who want there tachometer to work.

I really believe my car has been garaged its whole life, there is no evidence of heat damage anywhere and I’ve been through a lot of the car. You make good points about the life expectancy of the components and thanks for the info on how feasible it is to repair these. I am good for now as far as my Tach but if it does fail fail at some point in the future I may reach out to you to see how you’re doing with replacement Tachs.

[Wagon Master]

Check your tach against a good dwell/tach meter. If you can still find one.

[gbullman]

Check your tach against a good dwell/tach meter. If you can still find one.

I have a Heathkit Tach / Dwell meter that I built and have used for decades that I believe to be accurate. I’ll compare the in dash Tach to that when I get a chance. Also have a manufactured Tach / Dwell around that will be another comparison.

I’ll post results when I have them

[bbodie52]

I used to love Heathkit projects! My father introduced me to Heathkit when I was a kid, and that led me into an interest in electronics that ultimately put me into a long career in electronics maintenance and repair with the U.S. Air Force!

I used to have one of these units from Sears, but someone stole it!

Now this is my weapon of choice! The Innova 5568 professional timing light includes four advanced function digital readouts utilizing an easy-to-read backlit LCD. Built for one-hand use, the 5568 maximizes vehicle analysis efficiency and durability.

The Innova 5568 Pro Digital Timing Light is made from a heavy-duty, ABS, shock proof housing encased by a molded boot for extra protection against the engine fan blades and accidental drops. It features a backlit screen for easier viewing in hard to see places. The four function digital readout displays tachometer ranges from 249-9,990 RPM, Advance Degrees (2 and 4 cycle) 0-90 degrees, dwell (2-12 cylinders) from 0-180 degrees and voltage from 10-16 volts DC.

INNOVA 5568 Pro Digital Timing Light with Storage Case

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The newer lights also measure your rpm's and make it easier to set up your complete advance curve just in case you install a new distributor/a new vacuum cannister/new weights and springs etc.

And they are lot smaller than a SUN distributor machine.

[gbullman]

Brad,

My Heathkit Tach / Dwell looks the same as yours but the case is blue. I’ll get a photo tomorrow.