'65 c10 - I'd like my speedometer to read correctly

[Woogeroo]

my '65 stepside c10 lwb(127"wheelbase)

has a 350 sbc engine

a th350 transmission

and a stock rearend with about the 3.08 - with the posi/limited slip.

the plastic gear inside the tranny that attaches to the cable that goes to the back of the speedometer has 23 teeth on it.

I have not yet quite figured out the math on this to figure out what I need to do. I know the number of teeth on that plastic gear has something to do with it.

My truck also has the stock type 15 inch wheels.

With these tires:

P255/70R15 uniroyal gts

Now, is there something in the back of the speedometer that needs to be changed also?

Any help or moral support is appreciated.

-W

[60-66 Nut]

Woogeroo, normally what I do is just keep trying different gears until I get it close enough. But if you really want to get it right this might help you: http://www.bgsoflex.com/speedo1.html If you need to figure your exact tire diameter, they have a real good tire diameter calculator here: http://tech.oldsgmail.com/axle_RPM.html The gear in the transmission is called the drive gear and the gear on the end of the cable is the driven gear. It may be necessary to change the drive gear to get it real close. Sometimes it's hard to count the teeth on the drive gear in the trans. I had a chart that had the part numbers, tooth counts and color codes of all the available GM speedometer gears but I can't find it. If I find it I'll send it to you. Hope this helps.

60-66 Nut

[Woogeroo]

Originally Posted by 60-66 Nut

The gear in the transmission is called the drive gear and the gear on the end of the cable is the driven gear. It may be necessary to change the drive gear to get it real close.

60-66 Nut

60-66 nut - thank you so much for explaining that ... I was reading things about the drive and driven and I was like... AAAHAH!??

I have multiple threads with many posts going on everywhere - I think my brain might melt.. of maybe that is jsut the head cold I have... hmn.

http://www.stovebolt.com/bboard/cgi-...250;p=1#000007

http://www.chevytalk.org/threads/sho...D=#Post1357022

-W

[JRB]

W,
If you call your local speedo shop and tell them the gear ratio in your differential and the tire size on your truck then they will tell you and possibly even have the drive gear you need to put in the transmission end of the speedo cable. There is one bolt holding the speedo drive gear in position on the rear of the transmission. Unbolt that take the old drive gear out and slip in the new one. Hopefully all the gears are in the tail shaft of your transmission to make that gear work. I had to replace the tail housing on my 64 shortwide with one that had all the gears in it so the speedo would work. Wish you all the luck on your project.
Bruce

[Woogeroo]

JRB, thanks...

so in the phone book, what would I look under... 'speedometer shop' ?

-W

[Fred T]

Not too many speedo shops around anymore. Try an auto tranny parts store.

[RBW]

Dredging up an old thread here. I'm trying to get my TH350 speedo drive to give the correct reading on my '62 C10. It came with a 22 tooth driven gear. I used an iPhone speedometer app for reference. With the app at 25 mph, the truck's speedo indicated 18 mph (25/18). Tried a 21 tooth and came up to 25/20. Since a one tooth difference only changed the indicated speed by 2 mph, I skipped the 20 tooth and tried a 19. It ran 25/22. The lowest tooth count available is 18 and I figured it should give me 25/24, which would be close enough. But with the 18 tooth gear, the speed dropped back to 25/18 and the needle is waivering quite a bit. This makes me wonder if this smaller driven gear is not fully engaging into the teeth of the drive gear on the pilot shaft. Has anyone run into this problem, and have you had any success with shimming the driven gear to run deeper in the housing?

[geezer#99]

Originally Posted by RBW

Dredging up an old thread here. I'm trying to get my TH350 speedo drive to give the correct reading on my '62 C10. It came with a 22 tooth driven gear. I used an iPhone speedometer app for reference. With the app at 25 mph, the truck's speedo indicated 18 mph (25/18). Tried a 21 tooth and came up to 25/20. Since a one tooth difference only changed the indicated speed by 2 mph, I skipped the 20 tooth and tried a 19. It ran 25/22. The lowest tooth count available is 18 and I figured it should give me 25/24, which would be close enough. But with the 18 tooth gear, the speed dropped back to 25/18 and the needle is waivering quite a bit. This makes me wonder if this smaller driven gear is not fully engaging into the teeth of the drive gear on the pilot shaft. Has anyone run into this problem, and have you had any success with shimming the driven gear to run deeper in the housing?

Sometimes easier to start a new thread.
In your case, just get a ratio adaptor.
Look here in the 'speedometer world' section.
https://transmissioncenter.net/shop/

[phraza]

Originally Posted by RBW

Dredging up an old thread here. I'm trying to get my TH350 speedo drive to give the correct reading on my '62 C10. It came with a 22 tooth driven gear. I used an iPhone speedometer app for reference. With the app at 25 mph, the truck's speedo indicated 18 mph (25/18). Tried a 21 tooth and came up to 25/20. Since a one tooth difference only changed the indicated speed by 2 mph, I skipped the 20 tooth and tried a 19. It ran 25/22. The lowest tooth count available is 18 and I figured it should give me 25/24, which would be close enough. But with the 18 tooth gear, the speed dropped back to 25/18 and the needle is waivering quite a bit. This makes me wonder if this smaller driven gear is not fully engaging into the teeth of the drive gear on the pilot shaft. Has anyone run into this problem, and have you had any success with shimming the driven gear to run deeper in the housing?

Hi mate, I have a similar issue with mine (http://67-72chevytrucks.com/vboard/s...d.php?t=714864). I have just put a 18 brown tooth plastic gear in mine and it's still showing off, and the needle is bouncing a bit. Might have to either get a ratio adaptor as mentioned above or a new speedo cable. not sure yet

[RBW]

I put the 19 tooth back in and the speedo is now running stable, but 3 mph slow. I'll probably try the ratio adapter to bring it closer to a correct reading.

BTW, the first problem I faced was a speedometer that was sometimes not working at all or occasionally bouncing up and down wildly. I checked the cable and it was fine. I next replaced the driven gear, and it was better, but still not stable. I then replaced the driven gear housing (aka the bullet) and this stabilized the speedometer. I believe the erratic speedo needle was due a combination of wear on the driven gear shaft and the inner surface of the bullet.

I then tried lower tooth counts, and when I got down to an 18, the erratic needle problem came back. I've rarely seen a system with directly engaged gears where you can change one gear without changing the mating gear, as they are typically machined to mesh correctly as a set. The option to change the driven gear within a range of several teeth tells me this system probably has quite a bit of play in it. A bit too much wear on the parts, or going to the smallest available tooth count appears to take things beyond the limit of workable play and the erratic running occurs.

[akqj9]

not trying to hijack the thread, but my speedo leaks where it connects to the trans. A trans shop gave me a new little rubber seal but it still leaks, Any ideas? thanks