411 to 308 Speedo Gear Advice Needed

[Andy K.]

What gear replacements would I need to change my 1965 Roadster from a 411 to a 308 rear equipped with a close ratio M21 trans?
Left alone, my speedometer reads 50 MPH when my GPS says that I am actually doing 68 MPH.
Thanks for any advice that you can give.

[Edward J.]

Re: 411 to 308 Speedo Gear Advice Needed

Andy, I Believe that the brown 18 tooth driven gear is what you'll need , this is the gear you'll see in the housing on tail shaft, But there is a drive gear that is pressed onto the output shaft that maybe different, there are only two choices for the drive gear and I think with the 411 gear it going to be wrong. if you call one of the corvette venders they will tell you how many teeth should be.this gear will require to remove the trans. tail shaft housing.

[Mike R.]

Re: 411 to 308 Speedo Gear Advice Needed

This is correct, however the best way is to compare your speedo with GPS and figure out what the ratio is and get the closest driven gear and see how that works against the GPS. If you have radial tires for instance, the diameter is probably not exactly the same as the OEM bias ply tires and you would want different gear(s) than original. If you pull your trans and install the original drive and driven gears the speedo could read incorrectly. BTW my local chevy dealer was cheaper for these gears than any of the catalog places and doesn't charge shipping so you might check with yours.

Mike

Andy, I Believe that the brown 18 tooth driven gear is what you'll need , this is the gear you'll see in the housing on tail shaft, But there is a drive gear that is pressed onto the output shaft that maybe different, there are only two choices for the drive gear and I think with the 411 gear it going to be wrong. if you call one of the corvette venders they will tell you how many teeth should be.this gear will require to remove the trans. tail shaft housing.

[Wayne M.]

Re: 411 to 308 Speedo Gear Advice Needed

..... there is a drive gear that is pressed onto the output shaft that maybe different, there are only two choices for the drive gear and I think with the 411 gear it going to be wrong. ....

Well, almost ... for the 3.3% of the 1965's that were delivered with 4.56 final drive, the drive gear (on the trans mainshaft) was six tooth [rather than the 8 teeth of the other two possibles], and this matched with a steel driven gear of 20 teeth, for the 7.75 tires of the day.

[Edward J.]

Re: 411 to 308 Speedo Gear Advice Needed

This is correct, however the best way is to compare your speedo with GPS and figure out what the ratio is and get the closest driven gear and see how that works against the GPS. If you have radial tires for instance, the diameter is probably not exactly the same as the OEM bias ply tires and you would want different gear(s) than original. If you pull your trans and install the original drive and driven gears the speedo could read incorrectly. BTW my local chevy dealer was cheaper for these gears than any of the catalog places and doesn't charge shipping so you might check with yours.

Mike

Mike is correct, However the tailoring of the speed by 1or2 mph will not happen because of tire size. by adding or subtracting teeth on the driven gear this will change the speedo. about 4-5 mph per tooth.

[Joe L.]

Re: 411 to 308 Speedo Gear Advice Needed

What gear replacements would I need to change my 1965 Roadster from a 411 to a 308 rear equipped with a close ratio M21 trans?
Left alone, my speedometer reads 50 MPH when my GPS says that I am actually doing 68 MPH.
Thanks for any advice that you can give.

Andy------


You have 2 choices:

1) Replace the speedometer DRIVE and DRIVEN gears. Replacing the DRIVE gear requires removal of the transmission from the car and partial disassembly. The DRIVE gear you will need is no longer available from GM but can be obtained in the aftermarket;

2) Use a speedometer adapter of appropriate ratio in combination with a possible DRIVEN gear change.

[Richard S.]

Re: 411 to 308 Speedo Gear Advice Needed

great picture and explanation wayne. now i know why i have a 20 tooth aluminum driven gear and with 4:11 gears it reads about 10 mph lower than actual speed. are there any 18 tooth that will match up to the 6 tooth drive gear?

[Joe L.]

Re: 411 to 308 Speedo Gear Advice Needed

great picture and explanation wayne. now i know why i have a 20 tooth aluminum driven gear and with 4:11 gears it reads about 10 mph lower than actual speed. are there any 18 tooth that will match up to the 6 tooth drive gear?

Richard------


I would not expect the steel DRIVEN gear with a 4.11:1. Wayne was talking about its use for 4.56:1 applications. Maybe this car was originally 4.56:1.

In any event, if you have taken out a 20 tooth steel DRIVEN gear things get a little more difficult. I know of no 18 tooth gears ever available for use with the special 6 tooth DRIVE gear. There was once available a 19 tooth DRIVEN gear that I believe will work with this DRIVE gear. It was GM #3726645 but I don't think you'll find one.

You still have the options I described above except the speedometer adapter option would require a speedometer shop to come up with a custom adapter of the ratio necessary to convert your existing DRIVE/DRIVEN combination to the output speed required for a 3.08:1 gear ratio with the size tires you are using.

[Tom P.]

Re: 411 to 308 Speedo Gear Advice Needed

Andy------


You have 2 choices:

1) Replace the speedometer DRIVE and DRIVEN gears. Replacing the DRIVE gear requires removal of the transmission from the car and partial disassembly. The DRIVE gear you will need is no longer available from GM but can be obtained in the aftermarket;

2) Use a speedometer adapter of appropriate ratio in combination with a possible DRIVEN gear change.

FINALLY, someone hit the nail on the head!!!
There were two sizes of steel, pressed-on (drive) gears for the tail shaft----INSIDE the tranny. One is 1.84in OD and the other is 1.76in OD.
There are plastic DRIVEN gears which are easily changed out simply by removing the fitting on the side of the tail housing. The plastic gear range is (small diameter) 17-22teeth and (large dia) 22-25teeth.
The small driven gears are used with the large, 1.84 drive gear and the large driven gears are used with the small, 1.76 drive gear.
If you use a large driven gear on the large drive gear, MOST PROBABLY the plastic gear will be chewed up in a short time.
If you use the small driven gear with the small drive gear, it MOST PROBABLY will not engage, or at best, it will minimally engage for a very short period of time before the outer edges of the plastic wear down until it will not engage at all.
FROM THE FACTORY, the trannys installed in cars with a 3.70-down rearend will USUALLY have the larger 1.84 drive gear and will use the smaller driven gears.
And the trannys in cars with a 4.11-up rear WILL have the small drive gear and use the large driven gears. As I mentioned, there are (small) 17-22teeth gears and (large) 22-25 teeth gears. The LARGE 22teeth gear is green and the small 22teeth gear (if I remember correctly) is gray (some people like to call it silver).
Now, with all of that said, if a FACTORY installed tranny in a car that originally had a 4.11 rear, it will MOST LIKELY have the small drive gear and none of the small driven gears will work with it. Thus, as Joe mentioned above, the tranny will have to be pulled to swap out the 1.76in drive gear to the larger 1.84in drive gear, which of course requires removing the tail housing and that CANNOT be done with the tranny in the car!
Soooooooooooooooooo, again, as Joe mentioned, the EASIEST alternative is to obtain an adapter. These adapters have a nut on one end which screws right onto the speedo fitting on the side of the tranny (just as is on the end of a speedo cable). Then the speedo cable screws onto the other end of the adapter. Inside the adapter is a set of reduction gears, which can be tailored to increase or decrease the the outspeed of the speedo fitting.

Joe mentioned that the needed drive gear (1.84in) is no longer available. I buy ALLLLLLLLLLLLLL my 4sp parts, including speedo drive gears, from Larry Fischer of D and L Transmission http://www.dandltransmission.com/ .
There are of course other aftermarket tranny parts sources, but I have done business with Larry for over 30+yrs and he is great.

[Joe C.]

Re: 411 to 308 Speedo Gear Advice Needed

FINALLY, someone hit the nail on the head!!!
There were two sizes of steel, pressed-on (drive) gears for the tail shaft----INSIDE the tranny. One is 1.84in OD and the other is 1.76in OD.
There are plastic DRIVEN gears which are easily changed out simply by removing the fitting on the side of the tail housing. The plastic gear range is (small diameter) 17-22teeth and (large dia) 22-25teeth.
The small driven gears are used with the large, 1.84 drive gear and the large driven gears are used with the small, 1.76 drive gear.
If you use a large driven gear on the large drive gear, MOST PROBABLY the plastic gear will be chewed up in a short time.
If you use the small driven gear with the small drive gear, it MOST PROBABLY will not engage, or at best, it will minimally engage for a very short period of time before the outer edges of the plastic wear down until it will not engage at all.
FROM THE FACTORY, the trannys installed in cars with a 3.70-down rearend will USUALLY have the larger 1.84 drive gear and will use the smaller driven gears.
And the trannys in cars with a 4.11-up rear WILL have the small drive gear and use the large driven gears. As I mentioned, there are (small) 17-22teeth gears and (large) 22-25 teeth gears. The LARGE 22teeth gear is green and the small 22teeth gear (if I remember correctly) is gray (some people like to call it silver).
Now, with all of that said, if a FACTORY installed tranny in a car that originally had a 4.11 rear, it will MOST LIKELY have the small drive gear and none of the small driven gears will work with it. Thus, as Joe mentioned above, the tranny will have to be pulled to swap out the 1.76in drive gear to the larger 1.84in drive gear, which of course requires removing the tail housing and that CANNOT be done with the tranny in the car!
Soooooooooooooooooo, again, as Joe mentioned, the EASIEST alternative is to obtain an adapter. These adapters have a nut on one end which screws right onto the speedo fitting on the side of the tranny (just as is on the end of a speedo cable). Then the speedo cable screws onto the other end of the adapter. Inside the adapter is a set of reduction gears, which can be tailored to increase or decrease the the outspeed of the speedo fitting.

Joe mentioned that the needed drive gear (1.84in) is no longer available. I buy ALLLLLLLLLLLLLL my 4sp parts, including speedo drive gears, from Larry Fischer of D and L Transmission http://www.dandltransmission.com/ .
There are of course other aftermarket tranny parts sources, but I have done business with Larry for over 30+yrs and he is great.

That's the way I understood it to be. Going from a 3.70 to 4.11 ratio requires change-out of drive gear for proper calibration. 4.56 and 4.11 use same drive gear.

I brought my Muncie to Larry years ago to have him machine the main case idler shaft holes from 7/8" to 1" diameter in order to retrofit the larger cluster shaft onto a 1965 transmission. I did this as a result of the common stretched "pin" hole in the main case, and rather than bush it, I decided to upgrade to a stronger, later application cluster gearset and shaft.

Larry recommended moving the c/l of the cluster shaft a few thousandths closer to the main shaft which would ensure stronger gear meshing on a wider portion of the gear flanks. My first questions to Larry were whether this modification would cause extra heating, noise, and engagement problems. He told me that this mod was his idea, and that he had been doing it for quite some time. There were no issues with it. I let him go ahead with it while I watched. I brought the main case back home and assembled the transmission. I have had no issues with it over last 8 years. It shifts smoothly and quietly, and I like the peace-of-mind of knowing that the pitch diameter is further down toward the root of the flanks rather than the gear tip.

[Domenic T.]

Re: 411 to 308 Speedo Gear Advice Needed

Andy,
I think you will have another problem going to a 3:08 with a M-21 box.
You will use a lot of clutch in 1st gear getting started.
When I did that I went to a 2:56 1st gear tranny.
The 2:20 will work but at the expence of your clutch. Also flywheel weight comes into play with a 3:08 and the 2:20 1st gear your trans has.

DOM

[Tom P.]

Re: 411 to 308 Speedo Gear Advice Needed

That's the way I understood it to be. Going from a 3.70 to 4.11 ratio requires change-out of drive gear for proper calibration. 4.56 and 4.11 use same drive gear.

I brought my Muncie to Larry years ago to have him machine the main case idler shaft holes from 7/8" to 1" diameter in order to retrofit the larger cluster shaft onto a 1965 transmission. I did this as a result of the common stretched "pin" hole in the main case, and rather than bush it, I decided to upgrade to a stronger, later application cluster gearset and shaft.

Larry recommended moving the c/l of the cluster shaft a few thousandths closer to the main shaft which would ensure stronger gear meshing on a wider portion of the gear flanks. My first questions to Larry were whether this modification would cause extra heating, noise, and engagement problems. He told me that this mod was his idea, and that he had been doing it for quite some time. There were no issues with it. I let him go ahead with it while I watched. I brought the main case back home and assembled the transmission. I have had no issues with it over last 8 years. It shifts smoothly and quietly, and I like the peace-of-mind of knowing that the pitch diameter is further down toward the root of the flanks rather than the gear tip.

Joe,
I wasn't going to mention that here, since that is NOT an NCRS correct form of restoration. But I've had Larry do this (raise the cluster shaft centerline and bore to 1in shaft) for me on a few early (7/8) cases with ZERO issues.

[Joe C.]

Re: 411 to 308 Speedo Gear Advice Needed

Joe,
I wasn't going to mention that here, since that is NOT an NCRS correct form of restoration. But I've had Larry do this (raise the cluster shaft centerline and bore to 1in shaft) for me on a few early (7/8) cases with ZERO issues.

Same thing as putting an LT1 cam in an engine that originally had a 30-30. Well.....not quite, because the LT1 is an inferior, smog cam as compared to the 30-30, and represents a downgrade rather than an upgrade.

A better comparison would be the practice known as "lacquerthane", whereby a superior, more durable paint product is applied in a purposefully sloppy manner in order to emulate the slip-shod painting methods of the 1950's - 1970's................................no, that's not entirely correct, either.

I think that you can understand what I mean, however.

[Andy K.]

Re: 411 to 308 Speedo Gear Advice Needed

Would the M-21 trans have a 32 or 27 spline output shaft?

[John H.]

Re: 411 to 308 Speedo Gear Advice Needed

Would the M-21 trans have a 32 or 27 spline output shaft?

Andy -

27 splines, through 1970.