What speedo gear do I need?

[farside847]

So I finally got my gear set changed from a 3.73 to a 3.08 - my trans is a 3 on the tree. Unfortunately I have no idea what speedo gear I need to make it accurate again. Anyone know? How hard are they to swap?

Sorry for the dumn questions, but Ive never done this before....

Thanks!

[jhow66]

Read this:
http://www.tciauto.com/tech_info/speedo_gears.htm

Then just unscrew your speedometer cable at trans. Then remove bolt holding the speedometer gear housing in trans. Pull out housing. This is the "driven" gear. Look in hole to see what color "drive" gear you have. Then use the above to see which "driven" gear you need. Need some more help just ask.

[farside847]

Originally Posted by jhow66

Read this:
http://www.tciauto.com/tech_info/speedo_gears.htm

Then just unscrew your speedometer cable at trans. Then remove bolt holding the speedometer gear housing in trans. Pull out housing. This is the "driven" gear. Look in hole to see what color "drive" gear you have. Then use the above to see which "driven" gear you need. Need some more help just ask.

Thanks, Ive seen that site before but unfortunately they do not list my transmission type.

[jimfulco]

3-speeds generally use similar size gears as the smaller series of T-350 gears. You may not be able to tell the color of your drive gear due to fading or poor lighting, so here's another site that may help you out:

http://novaresource.org/speedo.htm
and the 2nd page:
http://novaresource.org/speedo2.htm

[farside847]

Hmmmm, what color do you think this is? Light green? The "driven gear" is a 22 tooth dark green one.

[farside847]

So, here is my math. My onld gear was a 3.73. According to the formula you do 3.73 x 20.2 / 28 (my tires are 28" tall) and you get 2.69. I know I have a 22 tooth driven gear now so I look on the chart and the closest thing I can find is a drive gear with 8 teeth.

So, if I do the math with my new gear of 3.08 it would be 3.08 x 20.2 / 28 and I get 2.22 - I look up that on the chart assuming my drive gear is an 8 and it says I need a 18 tooth driven gear.

Does that sound about right to you guys? For some reason I was thinking my drive gear had more than 8 teeth. Hell, I can see 6 in the picture.

So going from a 3.73 to a 3.08 I would go from a 22 tooth to a 18 tooth? I guess that sounds right, I need the speedo cable to move faster than it is now because my speed is about 20% low... Hmmmm.

I havnt done this much math in 15 years

[jhow66]

Try it and see--they don't cost much.

[skokie]

I went through this same ordeal on my t400. Here are the links that I have.
http://www.tciauto.com/components/speedometer_gears.htm
http://www.transmissioncenter.net/sp...n_______va.htm
http://www.bgsoflex.com/speedo1.html

[farside847]

Hmmm, grumble grumble. So I went and bought at 18 tooth driven gear (pn 3987918) and put it in. Now the speedo if more off than before! Plus a little more jumpy than before. I took it back out and took a good look at it, its the same length as the old one and the gear is slightly smaller in diameter. I put it back in and it did the same thing. Could this gear be too small in diameter and not be meshing correctly and only hitting ever other tooth?

[Longhorn Man]

I read somewhere that California will calibrate them for you?
Maybe call your inspection station and ask?

[Green Machine]

Just got mine corrected again. We changed 3 speed trannys to one with less miles. Both had 8 tooth internal drive gears so we did not change them. If I ran the smallest available 18 tooth driven gear on the old tranny, the speedo was real close. I am running 3.07 rear gears and a 235/75/15 tire.

The problem was that the newer tranny was run with the largest gear, a 22 tooth. That must have worn the drive gear enough that my 18 tooth driven would not hardly catch on the drive gear, speedo jumped and read way slow, the temp fix was to run a 19 tooth to get the speedo to turn smoothly.

The final fix was to remove and disasemble the transmission and install a 9 tooth drive gear and a a 20 tooth driven gear. Speedo is right on the money now. That way if I do small tire size change I can go a tooth or 2 either way to correct.

Yes the tranny has to came apart to do this, but it is pretty easy. Easier would be to get one of those little gear ratio changing boxes that mount between that tranny and the cable. BTW you have to look at the end of the gear to count the teeth, hard to do when installed !!!, can't tell from the outside, just color.

Basiclly thats the same problem I had, donor tranny had 3.73 rear gears, with a 8 and 22 combo. old tranny worked just fine with 18 tooth, but newer tranny must have been worn to much. I could have installed a new 8 tooth drive and I and I am sure that would have worked too, but with that combo the 8-18, if you ever go to a taller tire, they don't make a 17 tooth driven gear, but if you go to a shorter tire you will have adjustment left then. If there is a tire size change in your future consider what combo that may use. Hope that helps.

[jimfulco]

Sounds about right. My Suburban had 3.08's when I got it, and it had a green drive gear (9 teeth, I think), and a 20-tooth driven gear. With 255/70-15 Radial T/A's (29" tall), it read 50mph when the Shreveport PD radar trailer read 50.