Looking for a GM part number for a 22-tooth speedo gear for a 1967 with PowerGlide.
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Re: 1967 Speedo Gear-GM Part Number
Paul -
GM #3987922 (silver) 22-tooth; I also show a 22-tooth GM #3860345 (green) for the same application (don't know the difference). -
Re: 1967 Speedo Gear-GM Part Number
John and Paul-----
I'm just "peeking in" from my travels: I won't be home and "officially" back online until tomorrow evening. However, I'll "field" this one.
The GM #3987922 silver gear is a "small-series" driven gear designed for use with the "large" speedo drive gear. If the gear that was taken out is an 18-21 tooth gear, then the 3987922 should work.
The GM #3860345 is a "large" driven gear designed for use with the "small" diameter drive gear. This gear will work if your existing driven gear is a 23-25 tooth count gear.
The 22 tooth count driven gear is the ONLY driven gear which is available in both the large and small diameter series.In Appreciation of John HinckleyComment
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Re: 1967 Speedo Gear-GM Part Number
I think what I am looking for is the #3987922/silver. That has 22-teeth. And it is available from GM Canada for $4.00. Going from the stock tire size to P215/70R15 has left me 10mph low: my current reading of 70mph is actually about 62mph. Found that out yesterday when our C4 and C5 Club members told me I was going too slow at highway speeds. We agreed to cruise at 100kph (62.5mph) and they have cruise of course and exact digital displays. I was showing a steady 70 mph in my 1967. Our Police radar clicks in at 120kph (~75mph). But on the way home at 70mph (actually 60mph) without the Club a Police cruiser gave me the thumbs up and said get it on. Kind of embarrassing to be going too slow in a Corvette...The new speedo gears will be in tomorrow. I ordered a #3860346/23-tooth also. Just to get a true calibration. Thanks for the replies!Comment
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Joe....
Joe -
Hope you enjoyed your vacation tour (and got some good lobster)
Did Powerglides all use the same size DRIVE gear, or were there two different sizes over the years like the Muncies?Comment
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Re: Joe....
This site is "Chevelle" but it has all the transmission gear combos, GM part numbers and formulas necessary to do a calibration. In my case the correct gear derives from the formula:
N(driven)=N(drive) x (Axle Ratio) x (Revs per mile)/1001
My 1967 has Dunlop Axiom Plus tires...P215/70R15 with revs per mile of 770 (from Dunlop web site).
So from the formula above (23-tooth/black, 3.36:1 rear end) I get:
23 x 3.36 x 770 = 59.4mph actual for a 60mph speedo reading. I will install the 23-tooth gear later this week.
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Re: Joe....
John-----
Most Powerglides used the "large" DRIVE gear for use with the "small OD" 18-21 tooth DRIVEN gears, since rear gear ratios used with most Powerglides required these DRIVEN gear tooth counts. The only exception that I know of is for 1967 with L-36/L-68 and 3.55 or 3.70 rear gears. These used the "small" DRIVE gear and the "large OD" DRIVEN gears. Either of the 2 metal DRIVE gears used for most pre-68 applications (i.e. the "large" and "small" OD) will fit pre-68 Muncies, T-10s, Saginaw 3 speeds, and Powerglides. They'll also fit 68-70 Muncies, but they were not originally used for those applications.In Appreciation of John HinckleyComment
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Re: Joe....
Paul-----
The 23 tooth black DRIVEN gear is not compatible with the DRIVE gear used for 18-21 tooth DRIVEN gears or the 22 tooth SILVER DRIVEN gear. So, if your transmission currently has any of these DRIVEN gears installed, then it means that you have the "large" OD DRIVE gear currently installed you cannot use the 23 tooth DRIVEN gear. If you do, you'll strip the DRIVEN gear.In Appreciation of John HinckleyComment
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Thanks, Joe *NM*
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Re: Joe....
Joe,
I have a new GM #3987922 silver gear. It came in yesterday. What is in the transmission at the moment is 20 tooth/blue. If I understand correctly the silver driven gear is compatible with the existing drive gear? If yes, the $4.00 spent on the black gear is not a problem. The silver will give me a true reading of about 57mph at an indicated 60mph on the speedo. And from what I have read, GM calibrated speedos to read about 3mph high from true so this (silver) is a good compromise. Please confirm that silver is OK. Many thanks.
BTW I sold the 1979 to my younger Brother. Bit of a difficult decision after 17 years of ownership. But the 1967 is a fun car and I am enjoying it very much. Even with the PowerGlide it has adequate power for my type of driving over a wide rpm range.Comment
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Re: Joe....
Paul----
The silver 22 tooth gear will work and be compatible with your existing DRIVE gear.In Appreciation of John HinckleyComment
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Re: Joe....
Thanks Joe. I will post the before and after rpm/mph results of blue vrs silver. Really appreciate your help!Comment
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Re: Joe....
Joe,
The 22-tooth/silver is in and works perfectly. Where before I had 70 mph at a true 100kph (62.5mph) at 2800rpm with the 20-tooth/blue, I now have 62 mph at the same 2800rpm. Can't get much closer! Thanks to you and others for your help. I am very pleased with this outcome!Comment
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