My speedometer has never worked right. I replaced the speedo gear awhile ago and while it worked it was a bit "jumpy". Recently it stopped working altogether so I pulled the plastic gear out. I found that the gear teeth were ground off around the perimeter. What could cause this? Is there something wrong with the internals of the transmission? How can you tell if it is installed correctly? It seems like you just slip it in and hope it meshes with the internal gearing. I must be missing something here. Any suggestions?
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Re: Speedo gear 1962 Borg Warner
Richard------
There are several things which could cause this. The most likely is the use of incompatible driven and drive gears. Another possibility is incorrect installation of the DRIVE gear on the transmission mainshaft. For T-10s, 63-67 Muncies and 71-74 Muncies, the steel DRIVE gear is a press fit on the mainshaft and there is NO feature to index the gear to the exact correct position on the shaft (only 68-70 Muncies have this feature). So, unless care is taken during installation of the DRIVE gear, improper positioning of the gear can be the result.
As I say, though, the most likely cause of your problem is the installation of a DRIVEN gear which is incompatible with the existing DRIVE gear. Through 1970, there are two sets of DRIVE and DRIVEN gears. For the sake of simplicity, I will describe one set as having a "small" DRIVE gear used in conjunction with "large" DRIVEN gears. These combinations are used mainly with the higher numerical rear gear ratios. The other set of gears uses a "large" DRIVE gear in conjunction with "small" DRIVEN gears. If one uses a "large"-series DRIVEN gear with the "large" DRIVE gear, a problem like you have will result.
Some folks think that you can use any driven gear to correct speedometer inaccuracy. However, that's absolutely NOT the case. If the DRIVEN gear that you pulled out of the transmission is a 22 tooth green gear, a 23 tooth black gear, a 24 tooth yellow gear, or a 25 tooth orange gear, then I think gear incompatibility is what's at the root of your problem. Sometimes, incompatible gears will actually work for some period of time before they "strip out". For T-10s, 63-67 Muncies, and 71-74 Muncies, the DRIVE gear is steel and the driven gear plastic. So, if an incompatible driven gear is installed it usually won't damage the drive gear; the driven gear will be sacrificed.
If the gear that you pulled out was any other color, then I expect that the gear misalignment problem that I described earlier is the culprit. Unfortunately, to correct this problem you need to remove the transmission from the car and partially dis-assemble it.In Appreciation of John Hinckley -
Re: Speedo gear 1962 Borg Warner
Richard------
There are several things which could cause this. The most likely is the use of incompatible driven and drive gears. Another possibility is incorrect installation of the DRIVE gear on the transmission mainshaft. For T-10s, 63-67 Muncies and 71-74 Muncies, the steel DRIVE gear is a press fit on the mainshaft and there is NO feature to index the gear to the exact correct position on the shaft (only 68-70 Muncies have this feature). So, unless care is taken during installation of the DRIVE gear, improper positioning of the gear can be the result.
As I say, though, the most likely cause of your problem is the installation of a DRIVEN gear which is incompatible with the existing DRIVE gear. Through 1970, there are two sets of DRIVE and DRIVEN gears. For the sake of simplicity, I will describe one set as having a "small" DRIVE gear used in conjunction with "large" DRIVEN gears. These combinations are used mainly with the higher numerical rear gear ratios. The other set of gears uses a "large" DRIVE gear in conjunction with "small" DRIVEN gears. If one uses a "large"-series DRIVEN gear with the "large" DRIVE gear, a problem like you have will result.
Some folks think that you can use any driven gear to correct speedometer inaccuracy. However, that's absolutely NOT the case. If the DRIVEN gear that you pulled out of the transmission is a 22 tooth green gear, a 23 tooth black gear, a 24 tooth yellow gear, or a 25 tooth orange gear, then I think gear incompatibility is what's at the root of your problem. Sometimes, incompatible gears will actually work for some period of time before they "strip out". For T-10s, 63-67 Muncies, and 71-74 Muncies, the DRIVE gear is steel and the driven gear plastic. So, if an incompatible driven gear is installed it usually won't damage the drive gear; the driven gear will be sacrificed.
If the gear that you pulled out was any other color, then I expect that the gear misalignment problem that I described earlier is the culprit. Unfortunately, to correct this problem you need to remove the transmission from the car and partially dis-assemble it.In Appreciation of John HinckleyComment
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Follow up for Joe
Joe,
Your diagnosis seems right on. It always amazes me how you are able to do that without even seeing the car.
I pulled a 24 tooth yellow driven gear out of the transmission. It appears to be neatly trimmed down to a smaller diameter which would suggest to me that I need a smaller gear in there. The question is which one? Can you tell me which ones are of the smaller diameter? I think I need the one with the most teeth in order to match up with my rear end (which I believe is from a passenger car, definitely not stock).
Thanks
RichardComment
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Follow up for Joe
Joe,
Your diagnosis seems right on. It always amazes me how you are able to do that without even seeing the car.
I pulled a 24 tooth yellow driven gear out of the transmission. It appears to be neatly trimmed down to a smaller diameter which would suggest to me that I need a smaller gear in there. The question is which one? Can you tell me which ones are of the smaller diameter? I think I need the one with the most teeth in order to match up with my rear end (which I believe is from a passenger car, definitely not stock).
Thanks
RichardComment
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Re: Follow up for Joe
Richard-----
The following gears are likely the ones that are compatible with your speedometer DRIVE gear:
GM #3987918-------18 teeth, BROWN
GM #3987919-------19 teeth, NATURAL
GM #3987920-------20 teeth, BLUE
GM #3987921-------21 teeth, RED
GM #3987922-------22 teeth, SILVER
There is no gear in the series of gears compatible with your likely DRIVE gear which exceeds a 22 tooth count.
The problem is that I don't know if any of the above gears will produce an accurate speedometer reading. If you actually need a 23-25 tooth DRIVEN gear, then you will likely have to replace the DRIVE gear in your transmission. As an alternative, you could use your existing DRIVE gear, one of the above DRIVEN gears, and a speedometer adapter of the appropriate ratio. But, that's only if you actually need a 23-25 tooth DRIVEN gear.
If you can provide me with the rear axle ratio and your tire size, I MIGHT be able to tell you which, if any, of the above gears will produce a close-to-accurate speedometer calibration.In Appreciation of John HinckleyComment
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Re: Follow up for Joe
Richard-----
The following gears are likely the ones that are compatible with your speedometer DRIVE gear:
GM #3987918-------18 teeth, BROWN
GM #3987919-------19 teeth, NATURAL
GM #3987920-------20 teeth, BLUE
GM #3987921-------21 teeth, RED
GM #3987922-------22 teeth, SILVER
There is no gear in the series of gears compatible with your likely DRIVE gear which exceeds a 22 tooth count.
The problem is that I don't know if any of the above gears will produce an accurate speedometer reading. If you actually need a 23-25 tooth DRIVEN gear, then you will likely have to replace the DRIVE gear in your transmission. As an alternative, you could use your existing DRIVE gear, one of the above DRIVEN gears, and a speedometer adapter of the appropriate ratio. But, that's only if you actually need a 23-25 tooth DRIVEN gear.
If you can provide me with the rear axle ratio and your tire size, I MIGHT be able to tell you which, if any, of the above gears will produce a close-to-accurate speedometer calibration.In Appreciation of John HinckleyComment
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