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Another speedo gear question

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4-door Chevelle

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Discussion starter · #1 ·
I have read all the threads and am now more confused. I called my Chevy dealer and he said I have to get an external adapter/reducer. I would much rather change the drive and driven gear if possible. My speedometer is running about 10% faster than actual and I already have the 22-tooth driven gear in there which is the largest you can go to with this combo.

Here is my setup: TH350 tranny, 3.73 gears, 26" tall tire.

Thank you for your help !!!!!!!!!!!!
 
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#2 ·
If you search, there's a website available that tells you what drive and driven gears you need based on transmission type, rear gear ratio, and tire size. Then you can take that information down to a GM dealer and get the right gears. The gears are readily available. Speedometer repair shops also often stock the gears. The dealer you spoke with either doesn't know what he was talking about and/or was too lazy to help you.
 
#3 ·
Darren,
It is true that an adapter is available from the parts dept (or at least used to be) which will increase/decrease the speed of the speedo cable (there are, or used to be, several under or over drive adapters). Also, speedometer shops USED to be able to make these up for you. The adapter simply attaches to the speedo fitting on the side of the tranny tail housing, then your speedo cable attaches to the adapter.
I understand that you don't want to go that way, fine. Thus, choice number next is to pull the tail housing (REALLY, REALLY need to pull the tranny to do this) and change the DRIVE gear on the tail shaft.
MOST LIKELY you have an 8tooth gear on the main shaft.
There are TWO sizes of the 8tooth gear: Big-1.84in, Small-1.76in. The big 8tooth gear is only compatable with the SMALL driven plastic gears (17-22 teeth). The small gear is only compatable with the BIG driven plastic gears (22-25 teeth). YES, there are TWO 22 teeth driven gears: green and gray.
Also, there is a 7tooth drive gear for the main shaft, but I don't know if it is the 1.84 or 1.76in diameter.
IF, as I suspect, your tranny has the big (1.84in) drive gear, then you can replace it with a 1.76in drive gear which will mate with the big 22-25 teeth driven gears. In my opinion, this would be the better way to go.
Have I successfully confused the issue even more for you? :confused:
 
Discussion starter · #6 ·
Georgio69
Yup, I did all that but I was still sorting out large gear/small gear issue. It would be a little more work for the dealer so yea, I think they kinda blew it off. I was on the TCI website for info and thanks for providing the link.

DZAUTO
I actually did get that and this is the way I want to go. I think if I get the large 8-tooth drive gear (orange or black?) I can get the small 23-tooth driven gear (black?) for a 2.88 ratio which is real close to 2.897 which is 3.73 x 20.2 / 26. Did I get that right? Then do I need to change the speedo housing???
 
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Discussion starter · #8 ·
Does anyone know the part numbers:

large 8-tooth drive gear
small 23-tooth driven gear

Will this work with the Style I (smaller) driven gear housing?
 
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Discussion starter · #9 ·
Armed with info from here and Camaro's web site I got the parts ordered:

large 7-tooth drive gear pink #6271537
small 20-tooth driven gear blue #3987920

they will be here tomorrow so I can get that part of the project done this weekend!
 
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Discussion starter · #10 ·
Oops ... you may not have caught it (I didn't) but I said I needed an 8-tooth gear but quoted a 7-tooth gear part number.

large 8-tooth drive gear black #3980346

That baby was HARD to get off the shaft and I can only imagine will be just as hard to re-install. How do you guys get the drive gear "pressed" on the shaft???
 
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Discussion starter · #11 ·
OK, so I was able to get gears right away for my two setups, 26" and 29" tires on the rear using the pink 7-tooth drive gear and the right driven gears for the different tire sizes, 18-tooth and 20-tooth. These combinations aren't the "closest" but the gears were available at the dealer. This is all installed now and at least appears to be correct.

The actual closest combination is using a 8-tooth drive gear mentioned above and using a 21-tooth driven gear which is red - #3987921 and a 23-tooth driven gear which is black - #3980346 (which is on order) for the two tire sizes. This falls outside of DZAUTO's comments in his last post so if you Tom or anyone can help me understand that I would appreciate it!
 
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#13 ·
4-door Chevelle said:
OK, so I was able to get gears right away for my two setups, 26" and 29" tires on the rear using the pink 7-tooth drive gear and the right driven gears for the different tire sizes, 18-tooth and 20-tooth. These combinations aren't the "closest" but the gears were available at the dealer. This is all installed now and at least appears to be correct.

The actual closest combination is using a 8-tooth drive gear mentioned above and using a 21-tooth driven gear which is red - #3987921 and a 23-tooth driven gear which is black - #3980346 (which is on order) for the two tire sizes. This falls outside of DZAUTO's comments in his last post so if you Tom or anyone can help me understand that I would appreciate it!
Click to expand...
The Chevelle speedometer was designed to be calibrated at 1000 revolutions per mile so your goal is to match your tire size, differential ratio, drive gear and driven gear to get as close to 1000 rev/mile as you can.

With 3.73 and 26" tires:

driven drive cable

18 7 1125 +12%
20 7 1013 + 1%
21 8 1102 +10%
23 8 1006 <+ 1%

with 3.73 and 29" tires:

18 7 1009 + 1%
20 8 1037 + 4%
21 8 988 - 1%

If you can get close to 1% you are doing well.

Verle
 
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