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Discussion Starter #1
A number of years ago I replaced the original PG with a TH350 in my 66. Since then the speedo has always indicated more MPH than the car is actually going. Generally 10 mph faster. I've ignored it because I didnt want to mess with the speedo gear. In the last year I've noticed a small leak at the speedo gear/cable connection so I went ahead and pulled the driven gear and its housing (bullet). The drive gear is light tan (probably originally white) and looks like it has 5 teeth, maybe 6. The driven gear is the same color and has 19 teeth. The driven gear is 3/4" dia.
I have 25" diameter tires and a 3.08 rear gear. I found a couple of previous posts so I tried the sites mentioned.
The site http://www.5speedtransmissions.com/calculators.html says that I need a 12 teeth driven gear if the drive gear is 5 tooth and that I need a 15 teeth driven gear if the drive gear is 6 tooth.
http://novaresource.org/speedo.htm says that I need a 17 tooth driven gear and a 7 tooth drive gear.

After all that my question is: Who's right?
And how do I accurately count the drive gear teeth w/o removing the tail shaft housing?

Thanks
Max
 

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Mark one tooth with a sharpie and spin the output shaft and count the teeth.
 

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Give these guys a call. They will tell you exactly which gears you need, and they are not expensive (the ones for my T5 were $35 or so).

www.transmissioncenter.net


Dave
 

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According to your information you need a gear ratio of 2.488:1, using the novaresource site which I have used successfully in the past.

Your 19 tooth drive gear can not be used for this application because there is no combination of gears that come close to the ratio you need if you kept the 19 tooth gear.

My first choice would be the 2.5:1 (8 tooth driven/20 tooth drive) combo because it is the closest numerically to what you need.

Second choice would be the 2.44:1 (9 tooth driven /22 tooth drive gear).

Third choice would be 2.43:1 (7 tooth driven/17 tooth drive)

Some gear combinations are not available from GM or other suppliers, so just pick the ratio that is closest that is available.

HTH,

Dan
 

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Give these guys a call. They will tell you exactly which gears you need, and they are not expensive (the ones for my T5 were $35 or so).

www.transmissioncenter.net


Dave
X2, fixed me right up. Call them.......have your tire height and gear ratio handy......had the gears in two days.......my speedo is dead on the money now.....


T,
 

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X2, fixed me right up. Call them.......have your tire height and gear ratio handy......had the gears in two days.......my speedo is dead on the money now.....


T,
Do it right the first time then you will be happy. It is not that big of a deal to change these gears. You do not need any special tools. Some guys even do it with the tranny in the car (I took mine out).

Dave
 

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Discussion Starter #7
Thanks for all the responses.
OK, so I got under there again and did a recount of the drive gear... it has 8.
So I'm already pretty close with a 19 driven. Now the question is, how do you get the inner seal out of the (bullet) housing that holds the driven gear?
 

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Discussion Starter #8
Well, I ended up buying a new bullet (w/ seal) and put that in with a 20 tooth driven gear. At an indicated 60 mph it takes me 1 minute and 3 seconds to go from one highway mile marker to another, I did this repeatedly.
So I'm pretty close.
 

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That's about as close as you will ever get with these old cars. Back in the day we always had to check the speedo that way and then remember the correction.:rolleyes: Nice job!

Dave
 

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That's about as close as you will ever get with these old cars. Back in the day we always had to check the speedo that way and then remember the correction.:rolleyes: Nice job!

Dave
Very true. The factories usually errored on the side of reading fast. A speedo reading 1 - 3 MPH fast is pretty normal.
 

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You can have a speedometer recalibrated without changing transmission gears to be dead-on at 60 MPH even after changing rear gear ratio or tire diameter. When I restore a speedo or turn it into a 140 or 160, all the calibration is done to the speedometer. When police cars have their speedometers recalibrated, they don't change speedo gears. You could have just stayed with the speedometer gear you originally had in it if you wanted to.
 

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Sounds like a 21 tooth driven gear would put you spot on.

I use a GPS that shows MPH instead of the highway mile markers.
 
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