Does anybody know where I can get these, where the speedo cable attaches to the tranny on a 70 4spd, there are various colored gears (I've seen them somewhere) which can be used to modify the speedo reading.
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Re: speedo gear in the tranny, 70
The speedo cable on original trans configurations for your year attaches on the pass side toward the top of the tailhousing to a threaded coupling facing downward at an angle. The speedo gears are determined by tire size and rear end ratio, so to get the proper sixe these factors must be known. A couple of the gears are still available from Chevrolet, others can be obtained from trans rebuilders and/ or Corvette sources such as Corvette Central. The colors are co-ordinated with the number of teeth on a gear, which is tied to the gear ratio/tire size combination. Post the tire size you're using and the rear axle ratio if you need help in picking a "ballpark" gear -
Re: speedo gear in the tranny, 70
The speedo cable on original trans configurations for your year attaches on the pass side toward the top of the tailhousing to a threaded coupling facing downward at an angle. The speedo gears are determined by tire size and rear end ratio, so to get the proper sixe these factors must be known. A couple of the gears are still available from Chevrolet, others can be obtained from trans rebuilders and/ or Corvette sources such as Corvette Central. The colors are co-ordinated with the number of teeth on a gear, which is tied to the gear ratio/tire size combination. Post the tire size you're using and the rear axle ratio if you need help in picking a "ballpark" gearComment
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Re: speedo gear in the tranny, 70
John, check the thread on this subject on 10th May. In general you have two groupings of these colored plasic driven gears ; 4 for trannys with the DRIVE gear (on the trans mainshaft) for 3.08 to 3.55 differential ratios, and another 4 for the tranny drive gear used for 3.70 and 4.11 ratios. According to Joe Lucia, these gears may be still available from GM. The part #'s for the first group of 4 are # 3860342 (natural color -- 19 teeth, for 3.08 ratios and F70 x 15 tires); 3860343 (blue -- 20 teeth, for 3.36 ratio); 3860344 (dark red -- 21 teeth, for 3.55 ratio). There's another brown gear 3860341 (18 teeth), if your speedo is reading too low with the natural colored gear.
Second group: 3860345 (green -- 22 teeth, for 3.70 ratios) and 3860347 (yellow -- 24 teeth, for 4.11 ratio). There's 2 more driven gears to play around with; # 3860346 (black -- 23 teeth) and 3860348 (orange -- 25 teeth) This 348 gear may give more accurate speedo readings with 4.11 gears and F70 x 15 tires than the yellow.Comment
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Re: speedo gear in the tranny, 70
John, check the thread on this subject on 10th May. In general you have two groupings of these colored plasic driven gears ; 4 for trannys with the DRIVE gear (on the trans mainshaft) for 3.08 to 3.55 differential ratios, and another 4 for the tranny drive gear used for 3.70 and 4.11 ratios. According to Joe Lucia, these gears may be still available from GM. The part #'s for the first group of 4 are # 3860342 (natural color -- 19 teeth, for 3.08 ratios and F70 x 15 tires); 3860343 (blue -- 20 teeth, for 3.36 ratio); 3860344 (dark red -- 21 teeth, for 3.55 ratio). There's another brown gear 3860341 (18 teeth), if your speedo is reading too low with the natural colored gear.
Second group: 3860345 (green -- 22 teeth, for 3.70 ratios) and 3860347 (yellow -- 24 teeth, for 4.11 ratio). There's 2 more driven gears to play around with; # 3860346 (black -- 23 teeth) and 3860348 (orange -- 25 teeth) This 348 gear may give more accurate speedo readings with 4.11 gears and F70 x 15 tires than the yellow.Comment
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Re: speedo gear in the tranny, 70
Wayne----
The speedometer driven gears in your "first group" were discontinued many years ago. However, there are replacements for them which are identical as to teeth count and color code. These are:
GM #3860342= 3987919 GM #3860343= 3987920 GM #3860344= 3987921 GM #3860341= 3987918
Using these new numbers is important since the old numbers were discontinued so long ago, GM's current parts referencing might just show the old numbers as "not in file" and not supercede to the new numbers.
The speedometer driven gears in your "second group" are all still available under those numbers.
One other point which should be of interest to John is that 1970 was an "oddball" year for speedometer driven gears. For reasons known only to God and General Motors, in 1970 GM used speedomter adapters for many trans/rear ratio combinations. Although this is not shown in the AIM, it was the case. During the 68-72 period, speedometer adapters were generally used only for extremely high rear ratios(2.59:1) or extremely low ratios(4.56:1). However, for 1970-only, they were used for several different, common ratios. However, I recommend eliminating the adapter and going with the 1969-specified driven gear for the particular trans/rear end/tire combination. 1969 transmissions, rear axles and tires are the same as 1970, so I can't see why they wouldn't work equally as well on a 70.
Incidentally, in case you're not familiar with speedo adapters, they are a small, box-like device which attaches to the trans speedo fitting. The speedometer cable then attaches to the adapter in the same fashion as it would attach to the trans. Inside the "box" is a gear set which increases or reduces the trans speedo driven gear output speed by a specified ratio. The adapters used for 1970, GM #6477964 and 6577965 are still available but expensive(GM list about $60)and, in my opinion, unnecessary. Usually adapters are used in cases where precise speedometer calibration is necessary. An example would be calibrated, police-package speedometers.In Appreciation of John HinckleyComment
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Re: speedo gear in the tranny, 70
Wayne----
The speedometer driven gears in your "first group" were discontinued many years ago. However, there are replacements for them which are identical as to teeth count and color code. These are:
GM #3860342= 3987919 GM #3860343= 3987920 GM #3860344= 3987921 GM #3860341= 3987918
Using these new numbers is important since the old numbers were discontinued so long ago, GM's current parts referencing might just show the old numbers as "not in file" and not supercede to the new numbers.
The speedometer driven gears in your "second group" are all still available under those numbers.
One other point which should be of interest to John is that 1970 was an "oddball" year for speedometer driven gears. For reasons known only to God and General Motors, in 1970 GM used speedomter adapters for many trans/rear ratio combinations. Although this is not shown in the AIM, it was the case. During the 68-72 period, speedometer adapters were generally used only for extremely high rear ratios(2.59:1) or extremely low ratios(4.56:1). However, for 1970-only, they were used for several different, common ratios. However, I recommend eliminating the adapter and going with the 1969-specified driven gear for the particular trans/rear end/tire combination. 1969 transmissions, rear axles and tires are the same as 1970, so I can't see why they wouldn't work equally as well on a 70.
Incidentally, in case you're not familiar with speedo adapters, they are a small, box-like device which attaches to the trans speedo fitting. The speedometer cable then attaches to the adapter in the same fashion as it would attach to the trans. Inside the "box" is a gear set which increases or reduces the trans speedo driven gear output speed by a specified ratio. The adapters used for 1970, GM #6477964 and 6577965 are still available but expensive(GM list about $60)and, in my opinion, unnecessary. Usually adapters are used in cases where precise speedometer calibration is necessary. An example would be calibrated, police-package speedometers.In Appreciation of John HinckleyComment
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Re: speedo gear in the tranny, 70
Joe, You explained a lot (i.e. the adaptor), I have followed your responses for a long time and I'd give anything to know 5% what you know about these cars. I was surprised at the interest in this thread as I had expected to get flamed for asking, so here are some more details, 70 LT1, M20-3:55, P215/70r15 late May car. My speedo has been acting very erratic and at times only started working when shifting gears, finally it just stopped. I fooled around a little with the cable at the speedo and it started working again, so when it stopped again, I concluded that the problem was in the dash. Well the manuals says push on the clip and the cable will slide right out, after 29 years I don't think so! So out comes the dash and instrument panel, Take a spin around the block, no cable movement-it's broke, no-pull it out and it looks good. Start in at the tranny, pull out the adaptor, then the speedo gear (BTW it is/was dark red and I have a 3:55), clean up the adaptor, visually inspect the worm drive in the tranny (through the hole, and set to reassemble and the red gear drops into the tranny case! Having bad arthritis it slipped from my fingers, hopefully it won't hurt anything in there. I pulled out the exhaust, and tranny support brackets hoping to get more room to feel in the hole for it, but could not get it. What started as a simple little job ended up being 8 hours (i almost said the heck with it and started a frame-off) So when I get a new gear, Joe are you saying, eliminate the adaptor and connect directly to the case? I do think that the adaptor is/was the problem as it does not turn "easily".Comment
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Re: speedo gear in the tranny, 70
Joe, You explained a lot (i.e. the adaptor), I have followed your responses for a long time and I'd give anything to know 5% what you know about these cars. I was surprised at the interest in this thread as I had expected to get flamed for asking, so here are some more details, 70 LT1, M20-3:55, P215/70r15 late May car. My speedo has been acting very erratic and at times only started working when shifting gears, finally it just stopped. I fooled around a little with the cable at the speedo and it started working again, so when it stopped again, I concluded that the problem was in the dash. Well the manuals says push on the clip and the cable will slide right out, after 29 years I don't think so! So out comes the dash and instrument panel, Take a spin around the block, no cable movement-it's broke, no-pull it out and it looks good. Start in at the tranny, pull out the adaptor, then the speedo gear (BTW it is/was dark red and I have a 3:55), clean up the adaptor, visually inspect the worm drive in the tranny (through the hole, and set to reassemble and the red gear drops into the tranny case! Having bad arthritis it slipped from my fingers, hopefully it won't hurt anything in there. I pulled out the exhaust, and tranny support brackets hoping to get more room to feel in the hole for it, but could not get it. What started as a simple little job ended up being 8 hours (i almost said the heck with it and started a frame-off) So when I get a new gear, Joe are you saying, eliminate the adaptor and connect directly to the case? I do think that the adaptor is/was the problem as it does not turn "easily".Comment
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Re: speedo gear in the tranny, 70
John-----
First of all, I want to make sure that by "adapter", we're talking about the same thing. A speedometer adapter is a small, more-or-less rectangular "box". It has a fitting on one side like the fitting on the end of the speedometer cable. That female fitting threads onto the male fitting which retains the speedometer driven gear in the transmission tailshaft housing. Then, the speedometer cable female fitting threads onto the male fitting on the opposite side of the adapter.
Your 1970 LT-1 with M-20, 3.55:1 axle ratio, and F-70-15 tires, originally should have used speedometer drive gear(internal to the trans, on mainshaft)GM #3924097, speedometer driven gear GM #3860341(18 teeth, "brown" color code) and speedometer adapter GM #6477965.
Speedometer drive gear GM #3924097 is now known as GM #6261794. Speedometer diven gear GM #3860341 is now known as GM #3987918. Contrary to what I said in my previous post, speedometer adapter GM #6477965 is now known as GM #1565208(and currently GM lists for $101.67).
Your 215-R70-15 tires are virtually the exact same diameter as the original F-70-15, so no correction for tires is necessary. I don't know where the "red" speedometer driven gear came into the picture. This is a 21 tooth gear and would not be correct for your car under any circumstances that I can think of.
If I were you, I'd use the 69 speedometer gearing set-up for your trans/rear axle/tire combination. This would be speedometer drive gear GM #6261794, the equivalent thereof which you should already have in your trans(which is good news since this is a difficult one to change and if you had to change this you'd probably be better off sticking with an adapter even at $100). Then, use speedometer driven gear GM #3860345, which is a 22 tooth gear, color coded "green". Eliminate the adapter and attach the speedometer cable directly to the driven gear fitting on the transmission.
Of course, if you want to keep the car 100% original, you'll need to purchase a new adapter and driven gear as I described in the paragraph above which described what your car originally should have had. The problem is, the GM # 1565208 adapter is not configured exactly like your original GM #6261794, so it still won't be original. Like I said, I'd eliminate the adapter.
One more thing that I should point out is this: your speedometer drive gear(internal to the trans, on the mainshaft) is a nylon(plastic) gear, unlike the 63-67 models which used a steel gear which is practically indestructable. The nylon drive gears usually give good service and are problem-free. However, they are an organic material which is subject to degradation and deterioration over time, especially exposed to the hot, oily environment that they live in. It is not beyond the realm of possibility that this gear needs attention. I hope not, though.In Appreciation of John HinckleyComment
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Re: speedo gear in the tranny, 70
John-----
First of all, I want to make sure that by "adapter", we're talking about the same thing. A speedometer adapter is a small, more-or-less rectangular "box". It has a fitting on one side like the fitting on the end of the speedometer cable. That female fitting threads onto the male fitting which retains the speedometer driven gear in the transmission tailshaft housing. Then, the speedometer cable female fitting threads onto the male fitting on the opposite side of the adapter.
Your 1970 LT-1 with M-20, 3.55:1 axle ratio, and F-70-15 tires, originally should have used speedometer drive gear(internal to the trans, on mainshaft)GM #3924097, speedometer driven gear GM #3860341(18 teeth, "brown" color code) and speedometer adapter GM #6477965.
Speedometer drive gear GM #3924097 is now known as GM #6261794. Speedometer diven gear GM #3860341 is now known as GM #3987918. Contrary to what I said in my previous post, speedometer adapter GM #6477965 is now known as GM #1565208(and currently GM lists for $101.67).
Your 215-R70-15 tires are virtually the exact same diameter as the original F-70-15, so no correction for tires is necessary. I don't know where the "red" speedometer driven gear came into the picture. This is a 21 tooth gear and would not be correct for your car under any circumstances that I can think of.
If I were you, I'd use the 69 speedometer gearing set-up for your trans/rear axle/tire combination. This would be speedometer drive gear GM #6261794, the equivalent thereof which you should already have in your trans(which is good news since this is a difficult one to change and if you had to change this you'd probably be better off sticking with an adapter even at $100). Then, use speedometer driven gear GM #3860345, which is a 22 tooth gear, color coded "green". Eliminate the adapter and attach the speedometer cable directly to the driven gear fitting on the transmission.
Of course, if you want to keep the car 100% original, you'll need to purchase a new adapter and driven gear as I described in the paragraph above which described what your car originally should have had. The problem is, the GM # 1565208 adapter is not configured exactly like your original GM #6261794, so it still won't be original. Like I said, I'd eliminate the adapter.
One more thing that I should point out is this: your speedometer drive gear(internal to the trans, on the mainshaft) is a nylon(plastic) gear, unlike the 63-67 models which used a steel gear which is practically indestructable. The nylon drive gears usually give good service and are problem-free. However, they are an organic material which is subject to degradation and deterioration over time, especially exposed to the hot, oily environment that they live in. It is not beyond the realm of possibility that this gear needs attention. I hope not, though.In Appreciation of John HinckleyComment
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Re: speedo gear in the tranny, 70
Joe, I think we are talking about the same thing (i.e. the adaptor). One end is male with the same screw type thing as the end of the speedo cable it connects to the tranny. the other is 90 degrees and female the cable connects to that. Also there is a grease nipple. The male screw thing that connects to the tranny female (it contains the brown plastic gear, has the words AC Speedometer on it also there is a number on the body of the part 1583624 and under that it has 1:.833, which I assume is some sort of ratio. What do you think?Comment
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Re: speedo gear in the tranny, 70
Joe, I think we are talking about the same thing (i.e. the adaptor). One end is male with the same screw type thing as the end of the speedo cable it connects to the tranny. the other is 90 degrees and female the cable connects to that. Also there is a grease nipple. The male screw thing that connects to the tranny female (it contains the brown plastic gear, has the words AC Speedometer on it also there is a number on the body of the part 1583624 and under that it has 1:.833, which I assume is some sort of ratio. What do you think?Comment
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