How To - Speedo Gear Correction

[70GMCer]

Well, I finally decided to fix the speedometer on the 70 today. Ever since I converted to a 350 Turbo from a 3spd manual, the speedo has always read about 15 mph too fast. The following info and pictures are for a 2wd, 350 Turbo automatic trans.

I first had to do my research on what gears I needed for the change & what are the GM part numbers & gear colors. I went to this site http://chevelleengineer.home.comcast...eer/speedo.htm & it covers all GM trans with GM part numbers with colors.

I then went to this site to figure out what gears were needed. http://www.bgsoflex.com/speedo1.html

Keep in mine that our trucks & most others use the "1000 per mile revolution" speedometer.

The term "Driven Gear" is the gear that the speedometer cable connects to at the transmission & the term "Drive Gear" refers to the gear that fits onto the output shaft of the trans. Don't confuse the two........

You might get lucky & only have to change the "Driven Gear". No tear down of the tail shaft will be needed. In my case, I had a 22 tooth driven gear & a 10 tooth drive gear. This transmission had been installed in a mid 70's Impala before rebuilding so the rear end gears were much lower.

Turns out I couldn't go any higher than 22 teeth driven gear to obtain the desired speed correction. So, the calculation told me that with a rear end gearing of 3.73 & a tire height of 27.5", I needed a 19 tooth driven gear with a 7 tooth drive gear. You will have too know rear end gearing & tire height to do this.

The gears come in 2 different OD sizes. It seams the most common size is 1.84" OD. You have to make sure that you get a matched Drive & Driven OD size. This is noted at the 1st site I entered earlier.

1st, disconnect speedo cable from tranny. Easy enough!! Next put a drain pan under the trans tail shaft to catch tranny fluid that comes out. You will lose about 1 qt of tranny fluid, so make sure you have that to put back in.

Now remove the small bolt that holds the "Driven Gear Sleeve Housing" in the tranny. This housing just pulls out & has a rubber "O" ring on it. Be careful not to damage the "O" ring. This might come out with the driven gear attached. It not, just reach in and grab the driven gear & pull it out of the tranny.

Next, if you are going to have to replace the Drive gear on the output shaft, you are going to have to remove the drive shaft. Picture already shows drive shaft is removed.

Now, remove the (4) 9/16" bolts that hold the trans tail housing. The housing will come off easy, so remember to have the pan under to catch the fluid.

The colored speedo Drive gear is now exposed on the output shaft. Make sure that the transmission is in neutral. Grab the output shaft with your hand & rotate it to where you can see the clip as shown in picture #7. Look closely to where the screw driver is pointing. That is a clip that has to be pushed out toward the front of the tranny. You have to push on the 2 tiny prongs of the clip toward the output shaft & at the same time push the clip out all the way. Be careful as you will be needing this clip later.

The Drive gear will now come off the output shaft. All I had to do is grab, & pull. Pay attention on how the gear comes off in relation to the output shaft. There is a small hole in the shaft that has to align back up with the slot on the gear & where that clip goes back in as shown in picture #8

Picture #9 shows the new parts & old parts. I didn't replace the rear trans seal as I had just replace it before installing the trans about 200 miles back.

As shown in picture #10, you will have to tap on the new gear. Just tap around the gear lightly, & it will slide to where the old outline on the output shaft is. This is where you have to make sure that the little hole matches to the new gear. That hole is where the clip seats after reinstalling it thru the gear.

Before re-installing that gear clip, bend it back to an arch shape. When I remove it, it straightened it. If you don't bend it before putting it back in, it will be sloppy loose. You might even be able to get a new one.

Now just put everything back together. Look at picture #11 & that shows what position the Driven Gear Sleeve has to be. See the flat area on the sleeve? That's where the clamp/bolt tightens it up & holds it in place.

I found that GM dealers charge $8.00 on up each for gears. Here's a site where I bought my GM parts and the total for the 2 gears & "tail housing to case seal" was $14.59 shipped. Received them in 3 days. Here's the site. https://www.partszoneonline.com/index.php

I took my truck and checked the speedo with a GPS, and was dead on correct. This was an easy job & well worth the 1 1/2 hour spent.

[70GMCer]

More pics

[70GMCer]

Last pic

[70GMCer]

Originally Posted by gcburdic

Hey 70GMCer,

This is a GREAT write up....My speedo housing is leaking like a sieve.....did you do this on the ground???...or did you jack up the back of the truck to avoid losing alot of tranny fluid????

Yep.....jacked the back of the truck up to allow most of the fluid to run toward to front. Went to about 14" off the ground in the rear. Jack stands of course, with the floor jack under the rear end housing. Still ended up losing about a quart. Glad that's all I lost......