Speedometer Gear help!

[Ccass]

We get numberous inquiries about speedometer gears. Where to get them and which one to use for correct mph. Many are still available from GM but the dealer won't tell you that because they only cost a couple bucks and not worth the lookup time. We need an expert that has gone thru this process to help other readers. We saw an article in one of the GTOAA publications that tried to address this issue. Anyone have that article? It would be a very helpful post.

[Ccass]

We get numberous inquiries about speedometer gears. Where to get them and which one to use for correct mph. Many are still available from GM but the dealer won't tell you that because they only cost a couple bucks and not worth the lookup time. We need an expert that has gone thru this process to help other readers. We saw an article in one of the GTOAA publications that tried to address this issue. Anyone have that article? It would be a very helpful post.

[junkcollector]

Don't have the aforementioned artical but can still help some possibly. 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.

sorry I don't have any info on manual trans gear part numbers wish I did because I am going to need them soon for my Super T-10 maybe someone out there can help me out with them.

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

Thank you
Steve

[Scott Misus]

Steve:

Also, the speedo gear housings (the aluminum part that the driven gear rides in) are also different, depending upon how many teeth are on the driven gear. This housing, in turn, slides into the trans case and seals with an "O" ring. My memory regarding this isn't too good, so perhaps Tom Hand will be along shortly to help me out. Gach would probably know, too. But, chances are, HIS memory is a lot worse than mine, since he's WAY older.

Scott

[This message has been edited by Scott Misus (edited 01-28-2000).]

[Michael Farich]

http://www.corecomm.net/~faldrich/speedo.htm

[Bob Dillon]

An amazingly professional review of a problwem I've wanted to solve for a long time. Thanks to all. Not you, Misus.

------------------
I want to die peacefully
in my sleep like my grandpa,
not screaming like the passengers
in his car.