Which speedo gear do I need? buying, recommend, purchase, running

tom

My 69 Buick GS400 with auto now has a 3.42 rear, prior to that it had the 2.73.
Currently running 225/70/14 tires.

Anyone have an idea which speedo gear I would need to purchase to get the speedometer close to accurate and from where you recommend buying.

Guess this might be a common swap in gears...so I hope someone knows which gear (# driven teeth) I need to get and perhaps a part number.

Thanks
Rich

All the answers you need are here. Thanks to ANOTHER v8buick member that helped me. sp...n_______va.htm


Originally Posted by richnyMy 69 Buick GS400 with auto now has a 3.42 rear, prior to that it had the 2.73.
Currently running 225/70/14 tires.

Anyone have an idea which speedo gear I would need to purchase to get the speedometer close to accurate and from where you recommend buying.

Guess this might be a common swap in gears...so I hope someone knows which gear (# driven teeth) I need to get and perhaps a part number.

Thanks
RichWe need rear tire size, rear axle ratio, and the number of teeth on the speedometer drive gear.  That gear is on the output shaft of the transmission.  You can see it when you remove the driven gear sleeve.  If you get the back of the car up, you won't lose too much fluid when you pull it out.  Rotate the drive shaft with the transmission in neutral, and carefully count the number of teeth on the drive gear.  It's helical so go slow and count it a few times so that you are sure.  Then it is just plugging in the numbers and rounding off the result to the nearest whole number.  20168 divided by the tire height.  Take that result, and multiply by the rear gear ratio, and then by the # of teeth on the drive gear.  Divide the result by 1001, and round off.  For example.  For a car with 3.42's, P255/60R-15's (27"), and a 17 tooth drive gear, 20168/27 = 746.96, 746.96 X 3.42 X 17 = 43428.43.  43428.43/1001 = 43.38.  The correct gear is a 43 tooth gear

Thanks guys...I was in there yesterday and took a look at the drive gear. I was by myself and could not turn and look. The driven(speedo) gear is a 38 tooth that is in there now. I am running 225/70/14 tires, which calculates out to a 26.4" diameter. I am wondering...how many different drive gear sizes did they offer on the output shaft in 1969?

If I am calculating correctly with a 15 gear drive...I would need a 39 tooth driven gear and with a 17 gear drive...I would need a 44 tooth driven gear.


Originally Posted by richnyThanks guys...I was in there yesterday and took a look at the drive gear. I was by myself and could not turn and look. The driven(speedo) gear is a 38 tooth that is in there now. I am running 225/70/14 tires, which calculates out to a 26.4" diameter. I am wondering...how many different drive gear sizes did they offer on the output shaft in 1969?

If I am calculating correctly with a 15 gear drive...I would need a 39 tooth driven gear and with a 17 gear drive...I would need a 44 tooth driven gear.That is correct.  Most Buicks I have seen have 16, 17, or 18 tooth drive gears.  Some clip on, but some are press on.  You need to count the teeth.  If you have a 69 Chassis manual, it may tell you assuming you have the original transmission.  If you need a driven gear larger than 39 tooth, you need to buy another sleeve.  The sleeves have the compatible driven gears listed on them.  35-39, and 40-44

Thanks Larry...that clears up a lot of confusion on my part. I can see why  no one changed the speedo gear (when the rear was changed) as it would have (potentially) required to have gone back in there and change the drive gear on the output shaft if it was above 39...makes sense.

I checked my service manual but could not find much on the drive gear.

Thanks again
Rich


Originally Posted by richnyThanks Larry...that clears up a lot of confusion on my part. I can see why  no one changed the speedo gear (when the rear was changed) as it would have (potentially) required to have gone back in there and change the drive gear on the output shaft if it was above 39...makes sense.

I checked my service manual but could not find much on the drive gear.

Thanks again
Rich

It would be in the transmission section.  Some year Chassis manuals have it, others don't.  This is from the 1972 Buick Chassis manual.  I mean you need to use a different sleeve if it turns out you need a DRIVEN gear that is larger than 39 tooth, and that seems likely.  I would pull out the sleeve, and count the DRIVE gear teeth, then go from there.

Rich,         Here is a cut away view.  The DRIVE gear is green.  The DRIVEN gear is gray

64...speedogear.jpg

        Here is the sleeve with the DRIVEN(Red) gear inside.  Notice the gear numbers on the face.  This sleeve is for 34-39 tooth gears.

Ok, now I got ya. I was confused on the terminology. Did not know that was called the sleeve.  My sleeve is also for the 34-39 and my driven gear is blue(38 tooth). Had it out yesterday and could easily see the drive gear.

So in order to go to the anything above 39 you need to get a sleeve that is made for higher tooth sizes. First count the drive gear, then get a new sleeve(assuming it is above 39) and driven gear and I will be set.


Originally Posted by richnyOk, now I got ya. I was confused on the terminology. Did not know that was called the sleeve.  My sleeve is also for the 34-39 and my driven gear is blue(38 tooth). Had it out yesterday and could easily see the drive gear.

So in order to go to the anything above 39 you need to get a sleeve that is made for higher tooth sizes. First count the drive gear, then get a new sleeve(assuming it is above 39) and driven gear and I will be set.

That's it.   Hopefully your DRIVE gear is not larger than 17 teeth.  The largest DRIVEN gear is 45, I believe.  If you have an 18 tooth DRIVE gear, you'll need to change that out.

Rich,          Here are some GM part #'s

The sleeve for 40-45 tooth gears is 1362293

DRIVEN GEARS, 40 tooth (black) 1362048               41 tooth (yellow) 1362195               42 tooth (green) 1362049               43 tooth (purple) 1362196               44 tooth (gray) 9780470               45 tooth (light blue) 1372313

figured out I need the 44 tooth with the new sleeve. Ordered today from AC Delco...thanks