A 'while I am at it' task. My speedometer reads about 5-8 MPH higher then actual speed. I have a 20 tooth driven gear. I'm not sure about the ratio of the read end,(if someone can tell me if there's a number on the rear end I can cross to verify) but believe it 3:70 or so? Looking at the selection at Corvette Central, and given a guess at 3:70, I believe I need a 22 tooth? Does that sound reasonable?

Thanks and happy thanksgiving to all.

 

Here's the selection from Corvette Central

 

repost

 

FYI... the rear end will have different codes for SB and BB.
If you think you are 5-8 mph off, if you think you have a 3:70 and you think it needs the 22 tooth gear - your rear dif should have a date code that begins with letters FA... and you will be all set.

If it begins with FA, order the green 22 tooth.

If it doenst begin with FA, post it here and I am sure someone will know what gear is correct.

 

Look for the stamped numbers (ratio and date codes) on the bottom surface of the diff housing, between the spring and the strut rod bracket, and post them; that will tell us what ratio you have (assuming nobody has swapped different gears into the original housing) and we can give you the speedo gear you need.

 

After some cleaning, here's the code I think I see.

AM 6 27 67

Thanks

 

repost

 

Transmission ID is 3925661, vertical marking is P2P21A
I think this is a Muncie 21 wide ratio from a early 70's car?

Rear end gear ratio and date stamp is AM 6 27 67
I understand the date stamp, but need help with the AM ID for a gear ratio.

Thanks

 

AM is a 3.36:1 ratio.

 

gilmore1967,
You don't provide much information, therefore we can only guess at what you have, which it would appear that you have a 67.
The stamped code, AM 6 27 67, translates to 3.36:1 ratio, built Jun 27, 67. But that doesn't mean that 3.36 gears are ACTUALLY inside your rearend housing. A previous owner could have replaced them sometime in the past. And that is a genuine possibility since your car has a 72 Muncie in it, so it could have also had the rear gears changed.
The 3925661 casting number on the transmission main case indicates a 70-74 Muncie. The stamped date code, P2P21A, confirms that. It translates to:
P=Passenger Muncie
2=1972
P=Sep
21=day of month
A=M20 (wide ratio)
Therefore, your transmission was built for a 1972 GM product (not necessarily a Corvette until you provide the VIN stamped elsewhere on the main case). The first stamped 2 is for the YEAR MODEL, NOT necessarily the year of assembly of the tranny. Since your tranny has a Sep 21 date on it and it went into a 72 model vehicle, then it is MOST likely that the tranny was assembled on Sep 21, 1971 to go into an early production 72 vehicle. If you find the stamped VIN on the maincase, and it has a low serial number, then that would confirm that it was a very early production vehicle.

ALLLLLLLLLLLLLLLLLLLLL of the plastic driven speedometer gears are still available from Chev parts dept. GENERALLY speaking, adding or subtracting ONE tooth on the driven speedometer gear will change your speedometer reading ABOUT 5mph (up or down).
Here are all the part numbers/colors/teeth number:
3987917-purple-17teeth
3987918-brown-18
3987919-natural-19
3987920-blue-20
3987921-red-21
3987922-gray-22
The above are the smaller gears

3860345-green-22
3860346-black-23
3860347-yellow-24
3860348-orange-25
The above are the larger gears
MOST PROBABLY, your tranny used the smaller 17-22 teeth gears.

 

Thanks Tom. The car is a 67 with a small block. What I'm trying to do is to get the speedometer a bit more accurate. As my orginal post says, it is about 5-8 MPH on the high side. I have a 20 tooth gear and I want to try a higher gear I think to bring the MPH more in line with what I believe to be accurate speed.

I think there's the gears are more like a 3:70 or 3:55 or something like that. On the corvette central web page, there's a selection of gears for about $10 each I was wondering if I should get the 21 or 22 tooth gear? Also I see there's two 22 tooth driven gears.

Thanks

 

The 3987922 gray gear with 22 teeth is a SMALL gear.
The 3860345 green gear with 22 teeth is a LARGE gear.
The small and large driven gears are used with TWO different drive gears that are on the tailshaft. GENERALLY speaking, the big gear on the shaft and the small driven gears are used for cars with higher gears (2.73 thru 3.70) and the small gear on the shaft and the big driven gears are generally used for cars with lower gears (3.70 thru 4.56). Since the biggest majority of cars produced over the years (Corvettes as well as passenger cars such as full size, Chevelles, Camaros, etc) USUALLY had rear gears in the 3.08 thru 3.70 range, then the MOST COMMON driven speedo gears were the SMALL 20 thru 22 teeth gears. These gears are available from Chev for about $6-7ea.

 

The trans is a '72 M-20 wide ratio, so it has the "large" diameter DRIVE gear (all '71-up Muncies used only the "large" drive gear), so you have to use the family of "small" DRIVEN gears.

3.08 = 18-tooth, #3987918 (brown)
3.36 = 20-tooth, #3987920 (blue)
3.55 = 21-tooth, #3987921 (red)
3.70 = 22-tooth, #3987922 (silver)

From '71-up, 4.11 and 4.56 ratios used external geared adapters, as there were no "small" driven gears that would work with the "large" drive gear for those ratios.

 

That's absolutely correct! I was having a senior moment and forgot about that, since the 71-74 Muncies have have the large tailshaft, they can ONLY use the larger drive gear.

 

Thanks to both of you. I think I will pick up a 22 and 21 tooth LARGE gear at the chevy dealership and just try things. They are easy to change I have the wrong one.

The grear I have is green (or at least I think it is) Should I be able to match it up. I'm assuming I can see the difference between large and small gears.

Thanks guys. You folks are great.

 

NO, you need the 21-22 SMALL gears. As mentioned, the 71-74 Muncies ONLY come with the large drive gear, thus, only the small driven gears will mate with the large drive gear.

 

Got it. Thanks again.

 

Chevy dealerships sell those gears in pairs only at a price of about $12.50.

 

Hope you don't mind if I jump in here. I have a 64 coupe that reads about 15% high on the speedometer. At 70 mph on the speedo, actual speed is about 60 mph. The car came with 3.36:1 rear gears but the rear ratio calculates to be somewhere in the 3.6:1 area. The stamping on the housing is CB 9563. Which gear ratio does this indicate?

I have the muncie 4 speed, casting # 3839606 on the right hand side of the case. 1100846 is stamped on the rear flange. It currently has the 20 tooth gear with an outside diameter of .78". Is this a small or large gear? I'm guessing I need a 22 tooth gear.

Thanks for the help
Cheers
Steve

 

That's a "small" driven gear - they were 0.8" diameter, and the "large" driven gears were 0.87" diameter. Sounds like you need the 3987922 silver driven gear. The "CB" code says that housing originally came with a 3.36, but has been changed since.