The Living Nightmare of Speedo Gears

[StoningtonQB]

Last year my hands were tied between a new baby and moving out of town and I ended up buying a TH350 from Summit. I made the mistake of going to a friend's shop to put it in (instead of doing it myself) and I got it back, installed with no speedo reading. I mentioned this to him and he automatically assumed something unrelated and had already left for the day by the time I got down the street. Long story short I discovered the absence of the driven and drive gears inside the transmission. Conveniently, my old transmission was already gone (in spite of the fact I wanted it back) which had the accurate gears installed.

So I took the suggestion of some of you on the forum to buy the master kit on eBay. http://vi.raptor.ebaydesc.com/ws/eBa...27672000&ver=0 According to the seller, "For the drive gears: red 7, orange 8, green 9, and blue 10. For the driven gears: brown 18, white 19, blue 20, red 21, and grey 22." Since it's like throwing darts I installed the red set. At 60 MPH I'm reading 41 on the speedo. Before I go through the headache of dropping my yoke and tail shaft again, does anyone know the combination I should be using? I've tried using the various calc tools online but they really aren't telling me anything. I've got the standard 3.07 / 3.08 rear, sitting on 235/75R15s.

[MJN]

I know, another calculator but this one may help or a quick call to these guys. They helped me a ton with correcting my speedometer.
https://transmissioncenter.net/shop/...ar-calculator/

[C101972]

Google speed gear chart, I know TCI has a step through chart. Thanks Chuck
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[StoningtonQB]

What I don't know with these calculators is what to enter for my axle ratio. They want a number from 2 to 10.

Is there a rule of thumb where if you use less teeth it spins faster (MPH increase per tooth)? Assuming I want the least amount of teeth if my speedo is that slow, correct?

[MJN]

Not sure why you can't manually enter 3.07 or 3.08 if that is your gear ratio

[rockyrivermark]

Maybe I'm off base here but is the driven gear the one that goes on the torpedo that cable connect to?
if so I just pulled that piece out without removing the yoke and swapped the gear. took less than 2 minutes.
Again... one thing Ive not dug into is transmissions but just a thought.
Mark

[StoningtonQB]

Originally Posted by MJN

Not sure why you can't manually enter 3.07 or 3.08 if that is your gear ratio

I enabled scripts and it let me enter my ratio. But I'm still not understanding this... it says with 7 teeth and my ratio / tire size I need 28.87 teeth. There isn't a driven gear that has that many.

[68is4me]

Then try the 8,9 or 10 tooth in your equation till your get the right combination for the gears that you have.

[Wrenchbender Ret]

There are 2 different types of gears on these trans. The older ones had a bullet shaped holder with the driven gear smaller then the holder. It used about 16 to 23 tooth gears. The Latter models used a large aluminum holder with a gear of about 28-42 teeth.
That is probably what that listing is refering to. You evidently have the small one.
George

[pjmoreland]

I used this calculator:

https://www.tciauto.com/speedometer-gear-calculator

If I plug in your tire specs, it says your tire diameter is 28.88". If I enter that into the calculator, along with a 3.07 ratio and your red 7t drive gear, it says you need a 15t driven gear, which you don't have.

If I bump the drive gear up to the 9t, then it says you need a 19t driven gear, and you have both of those sizes.

[pjmoreland]

Here's another way to look at it. Your speedometer says 41 when you are going 60 (according to GPS I presume) with both red gears.

You need to increase the gear ratio by 60/41 or 1.463

Your current ratio is 7t/21t or .333

If you increase your .333 ratio by 1.463, then you get .333 x 1.463 = .488

The gears I mentioned above will give you pretty close to that.

9t/19t = .473

It will be off by about 3%, which is good enough in my opinion.

[my67c20]

I too had the same issue, tried the calculators, but did not feel confident in the data. So i downloaded an app called Speed Box, then swapped out the speedo cable gear until i got close enough. 2 weeks later something sprung and no more speedo. Now its about the tack or depending on where i am its about keeping up with traffic, or a bit lower. If i am really concerned, i just fire up the app.

[StoningtonQB]

Originally Posted by pjmoreland

Here's another way to look at it. Your speedometer says 41 when you are going 60 (according to GPS I presume) with both red gears.

You need to increase the gear ratio by 60/41 or 1.463

Your current ratio is 7t/21t or .333

If you increase your .333 ratio by 1.463, then you get .333 x 1.463 = .488

The gears I mentioned above will give you pretty close to that.

9t/19t = .473

It will be off by about 3%, which is good enough in my opinion.

Thank you this explained it perfectly. Yes I was holding 60 MPH using GPS. I vaguely remember my last speedo bullet having a white gear so this actually makes a lot of sense. Using this method I end up with green / white.