How to find correct speedo gear

[Reed]

Recently changed rear end ratio to 3:55 (I thinbk that's it) from 3:23. Which speedo gear do I need to get and where do I get one?Thanks

[Reed]

Recently changed rear end ratio to 3:55 (I thinbk that's it) from 3:23. Which speedo gear do I need to get and where do I get one?Thanks

[Blackcat]

What transmission are you using. I have a chart and gears for a th400.

[Ken K]

Which transmission do you have and what year is it? There are 2 different colors for a manual trans with a 3:55 rear ratio, red and natural. You can buy the gears new at a GM parts department for about $6.00.

[Ed Neasham]

Might as well jump on the bandwagon too. Ken, what color for a '65 M21 (close ratio) Muncie with 3.90 rear end?

[Reed]

I have an early muncie (65 I think).

['ol Pinion head]

Reed, It will also help everyone, if you can give us your tire size & wheel width. If those are not known, are you running shorter or taller tires than stock. The charts everyone is referring to, access stock sizes. IOW, how tall are your rear tires?

[Ken K]

The stock speedo drive gear for a 65 4 speed transmission is Red for a 3:55 rear gear and Black for a 3:90 rear gear ratio. The part number for the 3:55 red gear is #3987921 and the part number for the Black 3:90 gear is 9780523.

[Reed]

I never thought about tire size. I'm ruunning 14 x 7 wheels 235 60 series tires.
Thanks for all the replies, this board is great and the responses are helpful.
Reed

[Ed Neasham]

Ken, I too thank you for the information. Now that I've exposed my ignorance, I'll ccompound it by doing it again. I assume that the different drive gears are different because they have a different number of teeath. That would mean, that the driven gear would also be different too, wouldn't it? Do you order them as a pair, or what. Are they available somewhere, or do you need to go Junkyard hopping?

Ed

[Ken K]

The reason there are different colors is because there are a different number of teeth on the speedo drive gears. I think the number of teeth on the gear the speedo gear drives off of is the same but I am not shure, the gear is on the output shaft of the transmission. I think they designed it this way so the drive gears will interchange between different types of transmissions. The difference in the rear end ratios between a 3:55 gear and a 3:90 is only one tooth on the pinion gear, a 3:55 gear has 39:11 ring gear to pinion gear ratio and a 3:90 gear has 39:10 ring gear to pinion gear ratio. I have a chart for the 1964 speedo driven gear and there is 2 different speedo driven gear ratios for a 3:90 gear ratio, one is 18:6 for the green gear and 17:6 for the black gear.

[junkcollector]

I posted this response in a earlier topic about a similiar question. I hope this helps. Â*Â*Â*
According to The Car Builder's Handbook (isbn 1-55788-278-9) The gear part numbers for speedometer cable, TH350 and TH400 are as follows.
I. Gm drive gears (gear on end of cable)
Part Number No. of teeth
3987917 17
3987918 18
3987919 19
3987920 20
3987921 21
3987922 22
3987923 23
II. GM driven gears (the gear in the transmission that drive the Speedometer cable gear)
Turbo 350
Part Number No. of teeth
6261783 8
6261782 9
8629547 15
Turbo 400
Part Number No. of teeth
8629549 18
9629547 15
8440055 8
Now the question should be how do you determine the ones you need. Well according to the same book a American vehicles speedometer is calibrated for 1000 revolutions for every mile. So knowing this and a few other pieces of information which will be specific to your vehicle (tire circumference, axle ratio, and transmission final drive ratio which I believe for the TH350 and TH400 is a 1:1. Now the how to use this:
Determine how many revolutions your tire will make in 1 mile multiply that by your axle ratio then your transmission final drive ratio then multiply that by the driven gear and then divide by the drive gear. You will need to play with the numbers until you achive a final answer of 1000. Example: This is what I actually did on my truck to determine what speedometer gears I needed and it worked.
1 mile= 5280ft=63360inches
Circumference=piexdiameter
my tire diameter is 30inches
therefore the circumference=94.247
Now knowing this we can determine how many revolutions the tire makes in 1 mile.
(63360/94.247)=672.2704796
now multiply this by axle ratio mine is 4.10
(672.2704796x4.10)=2756.308966
now multiply this by the driven gear
(2756.308966x8)=22050.47173
now divide this by drive gear
(22050.47173/22)=1002.24169
which is as close to 1000 as I could get with factory gears.
Didn't mean to make this a long drawn out math lesson but am hoping to save some of you the time that I spent trying to figure all of this out and get it right.

If you have any questions regarding this please feel free to email me.
Thank you
Steve

[Richard]

I have a '63 Cat with a slim jim trans. Where can I get speedo gears for it? The one that is in there now overstates MPH by 20%.

[Michael Farich]

If you're referring to a '72 to '76 TH-375 trans then it uses the same gears as a TH-400.
BTW, there's a web site out there called "Chevelle Engineer" that has all the info on speedo gears and tire diameters but my old link won't work and I can't find a new URL with a search engine.

[MoJo]

Yes! JUNKCOLLECTOR- YOU DA MAN! Great info
Thanks

[Michael Farich]

Here's the link http://www.corecomm.net/~faldrich/speedo.htm

[Colorado Bill]

Wanted to update this topic with some new info. I corrected my speedo when I went home to visit my parents and my car around Christmas. The folks thought I was crazy, because I sat at the table doing math for a whole day.
I was able to use a ratio adapter with my original trans gears, but I also got some extra gears to use with taller tires and a different rearend ratio.

The GM parts guy and I found some adapters that are available (for about $45):

#3932220 (.7333 ratio, early '70's Vette)
#15636218 (.8974 ratio, '76 Monte Carlo)
(.9744 and 1.3 are also listed for
'76 Monte Carlo)
#15635472 (.8242 ratio, Chevy Pickup)
#15635469 (1.111 ratio)


Here's how I figured out what adapter and gear combinations to use (you can find tire rpm formulas in above posts and on the links below):

1) speedo revolutions per mile=
(drive gear X axle ratio X tire rev per mi)
/ driven gear

2) Divide 1000 by the speedo rpm to find what reduction adapter you need, or, multiply speedo rpm by a known adapter ratio, and use the one that gets you closest to 1000. (speedo rpm higher than 1000 makes the speedo read higher than the actual speed, lower than 1000 makes the speedo read slower than actual speed)
More info:
The Chevelle Engineer link above is helpful, as are the following: http://www.wallaceracing.com/speedo.htm
http://fteufert1.home.att.net/speedo/speedo.htm

This link has original Pontiac trans gear combinations and adapters. My dealer could not get the adapters to fit 3.90 and 4.10 gears in a '70 GTO automatic. As somebody mentioned above, know what part numbers you need when you go to the dealer--most of them won't have time to track down part numbers or figure out ratios. Hope this is helpful.