I'm getting ready to send my 65 transmission out for a rebuild and want to make sure I have the correct speed o gear setup. I have a M-21 gear set going into the case with 3.70 rear end. What setup do I need?
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Re: Need help with trans SpeedO gear
If you mean which plastic gear do I need, you need a 22 tooth gear for a 3;70 rear end.
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Re: Need help with trans SpeedO gear AND
Dennis tell your tranny guy that you rear is a 370, the speedo drive gear on the main shaft are different for 411 and up rears. Phil 8063Comment
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Re: Need help with trans SpeedO gear
Hi Dennis:
This topic can get complicated and I don't have all the necessary info, but I can tell you a few things to look out for:
1) There is a metal DRIVE gear on the tailshaft that turns a plastic DRIVEN gear in the tailshaft housing. These must be selected as a compatible set.
2) It is very important to get the right DRIVE gear because changing it requires disassembly of the transmission. Changing the DRIVEN gear can be done fairly easily with the transmission in the car.
3) The most common DRIVE gear has 8 teeth, but a 6 tooth version was used for some ratios such as 4.56.
4) I have read that there are actually two different versions of the 8 tooth DRIVE gear, with one being larger than the other. I do not know the details on this.
5) For an 8 tooth DRIVE gear, the most common DRIVEN gear for a 3.70 rear has 22 teeth.
If you want to do the math yourself for the exact tires you are running, it is not hard to do. The speedometer is calibrated for the speedometer cable to turn 1000 times per mile.
To do the calculation, start by measuring the circumference of your rear tire, in inches, with a tape measure. Divide the number of inches in a mile (5280x12=63,360) by the circumference of your tire to get the number of revolutions your tire will turn per mile. The answer will be around 740 revolutions per mile for a typical size C2 tire.
Now multiply by your rear axle ratio (3.70 or 3.73 ??) to get the driveshat revolutions-per-mile. The answer will be around 2750. Multiply this by the quotient of the DRIVE gear tooth count over the DRIVEN gear tooth count, such as (8/22 = .364). A driveshaft speed of 2700 revolutions-per-mile will be scaled by this factor to yield (2750 x 0.364)= 1001 revolutions-per-mile (remember, the target is 1000).
If your calculations show that a 22 tooth gear is not quite right for your tires, I believe that the other DRIVEN gears available for the 8-tooth DRIVE gear include 21 teeth and 24 teeth.Comment
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To add to info by others ---
.... there's even TWO 22-tooth plastic driven gears. This was not always the case; back in '60s only the green was available, and it matched with the 8-tooth steel drive gear 3708145 which has an OD of 1.750" and is 17/32 nds" thick. This drive gear was good for the 22 thru 25 tooth plastic gears ().788" dia). (Ive shown a 6-tooth steel drive gear for 4.56 final drives just for info, 'cause I don't have an 8-tooth handy). The 24 tooth yellow gear was the one for 4.11's with factory tires. A black 23 tooth and an orange 25-tooth are for fine tuning speedo calibration. These 22
The thickness (measured with caliper) is probably the easiest way to differentiate the two 8-tooth drive gears. For the final drives 3.08 thru 3.55, use 3708144, which is 1.85" OD and 0.610" thick, and mates with plastic gears 18 thru 21 teeth. This series of driven gears is 0.865" thick, which is why it will not fit on the larger diameter drive gear.
This may be a moot option, as you are going into the trans anyway, and could change to the proper steel drive gear, if you can find one.
Now if your car had the lower numerical ratio (large plasic gear) and you wish to convert to 3.70 (with larger steel drive gear), here's where the new 22-tooth (silver plastic) makes it easy, as it is of the small diameter grouping. The part # of this silver gear is 3987922. The other plastic gears are available on eBay.
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Re: To add to info by others ---
Hi Wayne:
Thanks for your detailed posting. I'm hoping that you can provide some more details on something that confuses me.
In my 1972 parts book, for 1967 4-speeds, the smaller diameter '144 drive gear is listed for "3.07, 3.08, 3.31, 3.36, 3.55, 3.70" axles, while the '145 drive gear is listed for "3.73, 4.10, 4.89" axles.
In other words, it appears that if the rear axle is a 3.70, the 22-tooth green driven gear can be used with either the '144 or '145 drive gears. Is that correct, and if so, why? One theory is that each of the driven gears is slightly different in diameter, and the green 22-tooth gear size is in the "sweet spot" that works with either drive gear.
This is not just an academic question because I recently changed my 67 from the factory 3.36 rear end to a 3.70 (it's a Powerglide car but the parts book issue is the same for 4-speeds). To correct the speedometer, I simply removed the blue 20-tooth driven gear and installed the green 22-tooth driven gear, without changing the drive gear. The new driven gear fit physically, but I won't know how it actually works until I drive the car in the spring.
I note that Dennis' original post was for a 65, and for some reason my parts book shows only the '145 for a 1965 3.70. I didn't think there was any physical difference in the speedometer gear arrangement between 65 and 67 4-speeds.
Can you shed any light on why, at least for 1966-67, the parts book says the green 22 tooth gear can be used with either the '144 or '145 dive gears? Is there a potential problem with my use of the green 22-tooth gear with the '144 drive gear?Comment
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Dealer Service Information Bulletin -- March '67
They explain that:
"new tire designs and sizes, and new axle ratios have increased the number of gear and adapter combinations required for all drive trains".
I believe that the low profile 7.75 x 15 tire differs in rolling circumference from the 7.75's of '65 and '66. Duke may be able to add his thoughts. I've copied one of the 2 charts in this bulletin. You can easily see the effect of tire size. I will define the codes for Corvettes from the 2nd chart that is used to interpret the chart below.
Driven gears:
1 = brown 16 teeth, 2 = natural 19 ", 3 = blue 20 ", 4 = red 21 "'
5 = green 22", 6 = black 23 ", 7 = yellow 24", 8 = orange 25 teeth
#s 1 thru 4 are smaller diameter family.
Drive gears: A = # 3708144; C = 3708145 (144 is larger dia.)
Large drive matches with smaller driven, and vice versa.
What does this chart say for '67 Corvettes ? [refer to 7.75-15 line] 3.08 ratio calls for 2A, or 19 tooth plastic driven and "144" steel drive. For 3.36, blue gear. So far so good. But for BOTH 3.55 and 3.70 they want the combo 5C, which is the green 22-tooth driven with the smaller "145" 8-tooth drive gear. Both of them cannot be perfectly correct. Either the 3.70 equipped car will read high on the speedo, or the 3.55-equipped car will read low. Why they don't specify the red 21-tooth gear (with "144") for the 3.55 tail is beyond me.
Continuing; for 4.11 ratios, go to "exceptions" at the right and 8C is the combo which is orange 25 teeth. (Ignore the 4.56 case for this thread)
For your situation, Joe, I would not have thought it possible to mesh the (larger) green 22-tooth driven gear on the (larger) "144" drive gear, without interference or forcing. That would be table combo "5A" which doesn't exist.
I think the problem is that the format of the parts catalog is too limited to show the proper requirements. My Oct '69 P&A30B shows gears same as the chart, but also shows '67 with different driven gears, but for LARGER tires. Example: for 3.55 & 3.70, use red gear with 8.45-15 tires. Under drive gears, my catalog also shows 3.70 using both large and small drive gears but doesn't link it to the driven gear, or calls for different tire size. This shortcoming of the catalog format is addressed by the charts, IMO.
What I would suggest is that "hybrid" silver/grey plastic 22-tooth driven gear # 3987922, which is in between (but closer to) the small diameter family than the large. Think it was introduced for this purpose; to allow those changing to 3.70 gearsets to keep the previous drive gear.
You can use the small drive/driven combo, and it will mesh until wear at the middle of the plastic gear causes slippage then rapid wear of the driven.
Whew !
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Re: Dealer Service Information Bulletin -- March '
Hi Wayne:
Thanks for your very informative response. In my case, the 22 tooth driven gear with an 8-tooth drive gear calculates to be dead-on for accuracy with the tires I am running. So, the only problem I need to resolve is which 22-tooth gear to use.
I'm 99% certain that the drive gear in my Powerglide is the '144 drive gear, and from your information it would appear that GM did not recommend using the green 22 tooth gear with that. It physically inserts without any undue pressure, but now you have me concerned about excess wear and little plastic particles circulating inside my transmission.
I think I will buy the new silver 22 tooth gear that you have suggested. From your photo it looks like this gear is similar in diameter to the blue 20-tooth gear that I removed, which was workng just fine.
Thanks again for your input on this complicated and arcane topic.Comment
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