Can someone give me an idea what color coded gear I need for the tranny [speedometer cable] with what little info that I have.
Speedometer reading 42 mph, GPS reads 55.
The following numbers are all I can find:
Tranny:
T10-7D
WL 2022
9 19 62

Rear end:
3707306
GM 3
B B 1 21 7

Tires:
225/70 R 15


Two full revolutions on the driveshaft = one and 1/4 on the tire.

I hope this limited info is helpful.

Thanks in advance,
Dennis

 

It may be of no help but I'll tell you what I've got. I'm using the same transmission and a 3.70 rear, my tires are 205-75 and I use a green driven gear, 22 tooth.

 

Those revs on the driveshaft and/or tire cannot be correct.

BUT....you're showing 42 on your speedo. The GPS is showing a real 55.

You need a gear with fewer teeth to spin the speedo quicker.

How many fewer teeth? 42/55 = 0.76.

That's your "factor"... 0.76.

So, if you have a 24 tooth gear currently.... 24 x 0.76 = 18.24, so an 18 tooth gear would correct your situation... pretty close.

 

I'm guessing that won't work without changing the gear on the transmission tail shaft. The diameter of the driven gear will be too small to engage. BTDT.

 

And that is EXACTLY correct!
FOR THE MOST PART, GM trannys, such as the 3sp, T10, Muncie, etc, used an 8 teeth DRIVE gear inside the transmission. These two 8 teeth drive gears came in 2 sizes, 1.76in diameter and 1.84in. These 8 teeth drive gears mated with 10 different plastic driven gears ranging from 17 to 25 teeth. (there were 7 and 9 teeth drive gears----------------BUT THEY ARE EXTREMELY UNCOMMON, AND IT IS EXTREMELY UNLIKELY YOUR TRANNY HAS A 7 OR 9 TEETH DRIVE GEAR)
DEPENDING ON THE REAR GEAR RATIO, THE COMBINATION OF DRIVE AND DRIVEN TRANSMISSION GEARS WERE DETERMINED.
The 10 plastic driven gears came in 2 sizes. The smaller 17-22 teeth driven gears mated with the larger, 1.84in drive gear.
The 22-25 teeth (and yes, there are TWO sizes of 22 teeth driven gears, gray and green) were a little larger and they mated with the smaller 1.76in drive gear.
THE SMALL DRIVEN GEARS MUST BE USED WITH THE 1.84IN DRIVE GEAR AND THE LARGER DRIVEN GEARS MUST BE USED WITH THE 1.76 DRIVE GEAR!!!
Soooooooooooooooooooo, with all of this said, before a determination can be made as to which driven gear to use, it is first necessary to know which DRIVE gear is inside your transmission, as well as which gear ratio is in your rearend

With all of the information above, as an example, if your transmission has the smaller 1.76in drive gear, and your rearend ratio is 3.55, there is NO plastic driven gear that will work to correct your speedometer reading. It would first be necessary to remove the tail housing of your transmission and remove the smaller 1.76 drive gear and replace it with a larger 1.84in drive gear. THESE DRIVE GEARS ARE MADE OF STEEL AND ARE PRESSED ONTO THE TAIL SHAFT OF THE TRANNY. NOT A PARTICULARLY EASY JOB FOR A NOVICE TO ACCOMPLISH! Once the drive gear is changed (IF that is what is needed) then a smaller driven gear (probably a 21 teeth driven gear) can be used to get the speedo to read correctly for a 3.55 rearend.
Are you thoroughly confused now?

 

When I changed the rear end on my car (4.11 to 3.36), I didn't want to go thru the trouble of replacing the driven gear in my transmission, so I installed a speedometer gear ratio adapter instead. For your situation, there is a 28% speed-up adapter available off-the-shelf for $88 which will get your speedometer reading in the ballpark (google transmission center). You can also have one custom made to your specifications for about $25 more.

 

Yea, totally confused which I have been most of my life. I'm beginning to think my best bet is to just bring my GPS along. The only draw back is my wife likes to hold it [the GPS of course] and lets me know when I'm over the speed limit.

Thanks for the info

Dennis

 

I'm beginning to think my best bet is to just bring my GPS along. The only draw back is my wife likes to hold it [the GPS of course] and lets me know when I'm over the speed limit.

Dennis[/QUOTE]

If you want to fix the car and not the wife, the adapter is the easiest way out here.

I agree with the others the gear on the tail shaft probably is too small for you to just drop 5-6 teeth on the driven gear.

 

This is the easiest solution to the problem and you can get a more exact reading this way. You should be able to find a place that can install the correct gears inside to get the necessary percentage difference.

 

easy-big drive gear little driven gear 22 teeth and below, little drive gear big driven gear 22 teeth and above. at 22 teeth it could be either configuration. First, You MUST KNOW what drive gear is in the trans. My 59 with a 3.70 posi rear and standard tires takes a small 22 tooth gray driven gear.

 

Me too. Only off gps 1 mph at 50. Only mine is listed as green, it's a pale green, but green. Eckler's description calls it green, 22 tooth. About $9.

 

AS I DESCRIBED ABOVE-----------------THERE ARE TWO 22 TEETH GEARS!
THE GREEN 22 TEETH GEAR IS USED WITH THE SMALL (1.76in diameter) 8TEETH DRIVE GEAR-----------THE GRAY 22 TEETH GEAR IS USED WITH THE LARGE (1.84in diameter) 8TEETH DRIVE GEAR.

Here is a comparison of the two drive gears.

 

I have a '56 that the driven gear seems to not turn. Maybe I have a small drive gear and a small driven gear. How to tell which drive gear without disassembly? Where to get a large driven gear?
Thanks in advance!

 

DZAUTO:
do you have pics of the two different 22 teeth DRIVEN gears?

also, my mistake, my driven gear in the 59 is light green with 22 teeth

 

Not trying to jump over Tom here, just giving him a hand.

Here are pictures of the ACDelco 22 tooth driven gears. The gray gear is part number 03987922 and the green gear is part number 03860345, courtesy of Summit Racing.

Also, here is a chart that shows the compatibility of the GM DRIVEN gears that go with the respective DRIVE gears.

Terry

 

The 17-22GRAY teeth driven gears mate with the larger 1.84in drive gear.

The 22GREEN-25 teeth driven gears mate with the smaller 1.76in drive gear.

Thanks for postinig the pictures showing the colors of each 22 teeth gear.