1967 Speedometer issues
I have a 327/350 hp 4 speed car with 3:55 gears. I guess I have two issues:1. This application calls for a red 21 tooth gear. A friend gave me one because I had a green 22 tooth installed. When a shop put the red 21 tooth in, it was torn up. After closer review I see that the red gear is about 1/8 smaller in diameter. I bought two other red gears, one at Carlisle and one from Paragon. They are both of the smaller diameter. I put the green one back in so that at least the speedo works. Any place I can find a correct red gear that is 7/8" diameter?2. My speedometer is sluggish. Upon take off, it doesn't register until I get to about 20 mph and then it "catches up" until I hit a higher speed. It then "catches up" again. Then when I slow down, it doesn't "catch up" to the slower speed. The shop says everything is hand tight. I haven't gotten under the dash/car yet. I don't "bend" as I used to!Any thoughts?ThanksJerry
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Re: 1967 Speedomter issues
Jerry,
This same thing has happened to countless Corvette owners. Not just Corvette owners. There are two drive gears (the one on the drive shaft) and there are two sizes of driven gears (the plastic ones) that you can change without much trouble. When a smaller gear is inserted the metal drive gear chews up the smaller plastic gear. It slips as it chews up the plastic gear and causes the speedo to jump around until the cable catches up and might smooth out when it attains speed.
The drive gear is either a 8 or 9 tooth gear and it is driven on the drive shaft. You are going to have to find a large 21 tooth red gear or change the drive gear which is a pain.
Check with some of the transmission rebuilders as they will recognize your problem and supply you with the gears you need. 4 speeds by Darrell? is just one that comes to mind!
JR -
Re: 1967 Speedomter issues
Jerry -
Your transmission has the "small" DRIVE gear pressed on the output shaft (1.77" O.D., GM #3708145), which means it was originally (or previously) paired with a 3.70, 4.11, or 4.56 axle; a 3.70 axle took the "large" 22-tooth green DRIVEN gear you removed earlier. There was no "large" DRIVEN gear combination for the 3.55 axle; correct speedo operation with a 3.55 axle requires the "large" DRIVE gear pressed on the output shaft (1.85" O.D., GM #3708144), which mates properly with the "small" red 21-tooth DRIVEN gear. You either need to have the DRIVE gear on the output shaft changed (which requires transmission removal), or have a speedo shop set up and install a ratio adapter box. The "large" and "small" output shaft-mounted DRIVE gears have the same tooth count (8) - they're just different diameters, to mate properly with the two different "families" of DRIVEN gears.Comment
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Re: 1967 Speedomter issues
Jerry,
If you have a parts book the driven gears are listed under group 4.337. It lists both the 3987921 21 tooth red gear for the 3.55 ratio with 7.75 tires and the 3860345 22 tooth green gear for the 3.55 ratio with 8.15 tires.
Under group 4.343 the parts book lists the drive gears and lists both the 3708145 with 7.75 tires and the 3708144 with 8.45 tires.
All very confusing so that is why I recommended you contact a transmission builder who knows what he is talking about when it comes to gear selection.
JRComment
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Re: 1967 Speedomter issues
JR------
The speedometer gear information contained in the Corvette P&A catalogs is notoriously inaccurate. Somehow, this information got "mixed up with" information for other Chevrolet vehicles. For example, 8.45-15 tires were never even used on any Corvette. There are many other similar inaccuracies.
For Jerry's 3.55:1 car, and assuming he has 7.75-15 or equivalent OD tires, he requires the "large" DRIVE gear, GM #3708144, and the "small series" DRIVEN gear of GM #3987921. However, as John mentions, he obviously has the "small" DRIVE gear, GM #3708145, currently installed. So, he either needs to change the DRIVE gear (which, of course, means removing and partially disassembling the transmission) or install a speedometer adapter. To go the latter route, he probably will need to take the car to a speedometer shop.In Appreciation of John HinckleyComment
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Re: 1967 Speedomter issues
Jerry------
If you search for a 21 tooth red DRIVEN gear that is 1/8" larger in diameter, you are going to be looking for a LONG time and you're still NEVER going to find one. As far as I know, no such gear was ever made. In fact, I don't think it is even possible to make such a gear that would also be compatible with your installed DRIVE gear. If it were, GM would have done it a long time ago as it would have simplified their speedometer gear installations.
There is only one tooth count of DRIVEN gears that is available in BOTH "small" and "large" series. That is the 22 tooth gear. In the "large" series it is the green gear you currently have installed, GM #3860345. In the "small" series, it is the silver gear of GM #3987922. In fact, for many years the latter gear (under ANY part number) was not available. GM came up with it in late 1970 as part of a "package" of changes to simplify their speedometer gear installations.In Appreciation of John HinckleyComment
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Re: 1967 Speedomter issues
Joe L.,
As I said, very confusing and the best recommendation would be to contact someone that builds transmissions and has old gear and possibly some new ones that would help Jerry. And yes I know that 8.45 tires were never installed on 67 Corvettes.
Somebody that has knowledge needs to look at this trans. hands on!
Don't you agree,
JRComment
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Re: 1967 Speedomter issues
For a 70 Corvette (F70-15 tires) with close ratio transmission , the parts book lists the 3860345 green 22 tooth gear for both the 3.55 and 3.70 rears without adapters. There is only a 4% speed difference between the rears, and the difference was just accepted by GM as speedometer error. Keep the green gear. It's the design gear for either rear.Comment
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Re: 1967 Speedomter issues
Jerry-----
By the way, how do you know that you have 3.55:1 rear gears? No 1967 L-79 with M-21 was ever originally built with such a gear ratio. I really don't see why anyone would have changed gears from 3.70:1 to 3.55:1. Is this a case in which someone just changed the whole rear assembly and the one they used happened to be a 3.55:1? In other words, do you know that the differential assembly is not original to the car?In Appreciation of John HinckleyComment
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Re: 1967 Speedomter issues
Joe,My tank sticker says the car has the 3:55 gears and the casing has the code "AN" and the build date of 1/20/67. The build date of the car is 2/8/67. So I am assuming it was built with that rear end. If someone put another rear end in, it would be quite coincidental that even the dates would match with the build date of the car. I have not done the "physical" test yet on the car.But that being said, would this gear issue be created just by the wrong toothed gear? I thought the speedo would be slower with the 22 tooth gear.Thanks,JerryComment
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Re: 1967 Speedomter issues
Per John Amgwert's Corvette Spec. Guide, '67 L-79 with a wide-ratio four-speed (M-20) had a standard 3.36 axle, and 3.55 was optional. With a close-ratio four-speed (M-21) 3.70 was standard and 4.11 optional.
If the 3.55 gear was ordered with a WR four-speed, a different WR transmission - one with the larger internal gear would have been installed than with the base 3.36 axle, so something doesn't add up.
The problem you report of the speedometer being slow to react to speed changes has nothing to do with the gearing. (You said nothing about steady speed accuracy.) It's probably a problem in the head - dirt or wear, which is causing excessive drag on the spindle, You should remove, clean, and lubricate the speedometer cable as as first step. I would bet it has not been serviced since the car was built.
If the above doesn't work then the IP will need to come out and the speedo removed and sent to a competent speedometer shop for service.
You should also audit the installed parts - numbers, driveshaft/halfshaft turns to verify installed axle along with finding the revs/mile spec for the installed tires. This infomation can be used to dial in speedometer accuracy. I believe the standard speedometer calibration is 1000 cable revs/minunte at 60 MPH, however the speedometer shop can change the magnetic flux on the speedo head to offset any error based on the closest speedometer gear that you can use along with verified axle ratio and tire revs per mile. I've had this done on a couple of speedometers because of using a higher revs per mile replacement tires, and they are dead nuts on.
DukeLast edited by Duke W.; September 10, 2014, 11:45 AM.Comment
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Re: 1967 Speedomter issues
Joe,I pulled out my tank sticker and it states an M20 versus an M21 as you mention in your quote. And someone else commented that the 3:55 gears were optional. Is that speedometer adapter mounted on the outside of the tranny? if that is so, can it be installed by a non=speedo shop?I'll try to find a shop around the Hartford, CT area.ThanksComment
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Re: 1967 Speedomter issues
Jerry------
Yes, for some unknown reason, I thought you had initially indicated that the car had an M-21. After I re-read you initial post, I discovered that you had not said that. So, now we know, for sure, that your car was equipped with an M-20 and, for that transmission, the 3.55:1 was an optionally available ratio. So, both the transmission and the rear end are likely original to the car.
The above being the case, I'd say it's likely that your transmission was originally built with the "small" drive gear. That being the case, it was also likely originally built with the 22 tooth green driven gear as you found. As Patrick mentioned, for 1970 (and 1968-69 as well) cars with 3.55:1 rear gears used the 22 tooth green driven gear. It is true that 1967 used 7.75 X 15 tires whereas 1968-70 used F-70-15 but those two sizes are very close in OD and revs-per-mile. While information on 1967 is a bit more sketchy, it seemed to me that 1967 with 3.55:1 used the 21 tooth red gear, but maybe not. With your currently installed drive gear,and assuming it's original, your car could not have originally been fitted with the 21 tooth red gear.
So, it boils down to this: are you sure that your speedometer calibration is actually off with the 22 tooth green gear? This can be easily checked with a hand-held or portable GPS. Don't expect it to exactly match the GPS since the GPS is very accurate and, unless one is real lucky, there will be some degree of inaccuracy with a factory speedometer, maybe 2-4%.
As Duke mentions, you may actually have a problem with the speedometer instrument. Some of your symptoms do sound like that. So, maybe your first shot should be involved with removing the instrument and having it serviced by a competent shop. Even if it's not the source of your problem, you won't have wasted any money since a speedometer really does need to be serviced from time-to-time and yours probably never has been.
As far as a speedometer adapter goes if you get to that, it does mount on the outside of the transmission. It simply screws onto the transmission's speedometer fitting and the speedometer cable then screws onto the adapter. Installing one is simple. However, selecting the right DRIVEN gear (in your case 22 tooth green to 25 tooth) to work with the available adapter ratios is not so simple. So, that's why I suggest having a speedometer shop do it.In Appreciation of John HinckleyComment
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Re: 1967 Speedomter issues
Joe,Thanks for the confirmation and the info. I think this is a winter project because I did replace the speedo cable last year.Thanks to everyone who responded. I'll keep you posted!JerryComment
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