Beltman

I changed the gears in my rearend and now my speedometer is way out of wack. What can I do to correct? Thanks

snowballs442

Speedometer shop can easily fix that or you can change plastic gears and the cable.

507OLDS

There is a plastic drive gear,inside your transmission,that is calibrated for different gear ratios.You just need to get the correct one for the rear gear that you have now.

Aceshigh

I need to do this too, so I got a question too.

This site here shows the formula for picking the proper trans gear for the speedo.
http://novaresource.org/speedo.htm
Gear ratio x 20.2 divided by tire height = Gear size
So 3.73 x 20.2 = 75.346 / 27" = 2.79

Small Speedometer Gear (3/4" diameter) = 22 (closest to 2.79)

Large Speedometer Gear (1-1/2" diameter) = 39 (exactly 2.79)

So I'm not sure which I need??? Is it the grey gear #22 (small) or brown gear #39 (large)??
http://www.tciauto.com/Products/Tech...eedo_gears.asp

I have no idea what this means.....

Boldsmobile

One fits gears from 34 - 39 teeth and the second one fits gears from 40 - 45 teeth.

Boldsmobile

The one you needs depends on which drive gear you pick to use with the driven gear.

Rickman48

I had a cop clock me w/radar as the speedo said 60 - he said 66 which was 10% slow on the speedo!
Pulled the small [driven] gear, counted teeth, [20 I think] went to the dealer and got an 18 tooth, installed it, and was right on!!
4-speed w/a 3:08 I think!!

Aceshigh

Bear with me here, I'm kinda a newb to this dept.

Using this pic for reference.
The Drive gear is the internal green gear correct??
The Driven gear is the white side gear I can remove correct ??



So are you saying I have to pick BOTH a Drive gear AND a Driven gear to get my speedo to function correctly or can I just buy a Driven gear ??

I have no idea how I'd change the Drive gear if I had to.

Boldsmobile

No problem.



Correct

Quote:

Originally Posted by Aceshigh

So are you saying I have to pick BOTH a Drive gear AND a Driven gear to get my speedo to function correctly

Correct

Quote:

Originally Posted by Aceshigh

or can I just buy a Driven gear ??

Only if the drive gear you have is the one you need.

Quote:

Originally Posted by Aceshigh

I have no idea how I'd change the Drive gear if I had to.

Pull the drive shaft. Pull the four bolts on the transmission tail shaft. That silver clip on the drive gear is what secures it to the tail shaft. Those clips are know to break.

The bolt in the pic next to where the speedo hooks up holds down a clamp. When you remove the bolt and clamp you can pull the driven gear housing and the driven gear. There are two types of housing to choose from and the one you need is based on which driven gear you have. I dont have sample of both otherwise id take a pic and show you.

Hope this helps.

Aceshigh

THanks,

Okay so now I need to determine what the stock tire size was for a 1970 Olds Cutlass.
I have 14" OS III rims and SVNT442 says they were 225/70/14's.
I already know my gears were 2.78 from the factory.

So if I use this Bowtie Overdrives calculator to see what my drive gear is
It says my Drive gear is a 13 tooth, and my driven gear is a 28 tooth most likely.
Final # is 2.1538462

So with 3.73's and a 285/40/18 rear tire using the Bowtie Overdrives calculator
I will need a 10 tooth Drive gear, and 28 tooth Driven gear.
SO naturally......I have to take off the trans tail housing to swap the driven gear. Figures......
Speedometergearchart22.jpg

Redog

I think the more important question is, which transmission do you have?

If you have a 200r4, it's better to just get a slow down or speed up adjuster

Run to Rund

Using the example above:
60/66 = .909
18/20 = .9
Do the ratio for your situation and if a gear with the needed number of teeth is offered by GMPD, you are good to go with just the driven gear.

Aceshigh

^ ^ ^ No idea what any of that is from.....^ ^ ^

If you're asking me, Th350

Redog

OK then thats easy enough to figure out.

What gear did you have in the rear? What new gear do you have in the rear? What plastic gear is in the tail housing? What color is the gear?

Take it out of the housing and count the teeth. Then I can look up what the other gear is inside the tranny and figure out which little tranny gear you need. Keep in mind you'll lose a bit of tranny fluid doing this

BlackGold

If you go the route of trying to figure out exactly which drive and driven gear you need based on tire size, etc, you will go through a lot of hassle and probably get it wrong the first time and have to try again.

If, on the other hand, you go the route of adding an "adapter" (gear reducer) between transmission and speedo cable, you will get it right the first time. Just measure what percentage slow or fast your odometer is (use mile markers over 10 or 20 miles on the highway) and get a reducer which corrects that percentage. Done.

Boldsmobile

Is there any place that offers an adapter at a reasonable price?