Technical GM T-5 speedometer gears

I have a 86ish S10 T-5 in my 37 and about ready to get the speedometer reading as close as I can get so I was wondering if those gears are still available at the dealer and are all GM speedo gears interchangeable between manual and automatics in the 70s and 80s. First time dealing with GM speedo gears.

 

take the gear out and look at it.... most of the manual transmission and Chevy TH350 gears were the small one, and are interchangeable.

according to this page, they are the same.

http://www.thegearbox.org/catalog/item/3819132/3532517.htm

 

I still have to see how many teeth are on the existing gear and if I remember correctly it had some kind of little gear box attached where the speedo cable goes.

 

the little box...look at it, there is usually a ratio stamped on it.

 

Going to look at it this weekend or early next week,the speedo is 9 MPH slow so it could be doing better then I think on the gas mileage but seems to be using more then a six should. Could the gears in the box be changed.

 

yes, there are lots of ways to fix it....you might want to first compare the odometer reading to a known measured mile (or better yet average over 10 miles), then find what percent off it is, which direction, and go from there.

 

Right now I am just using those your speed is digital signs along a couple roads to see how far off it is.

 

First off, take the cover off where you replace the driven gear. Find out what color the DRIVE gear is on the shaft inside the tailhousing. Purple is 7 tooth, red is 9 tooth. If you tell me your rear end gear ratio, rear tire dia I can try to find you a driven gear that is right.

 

3.55 gears and 235-75-15 on the tires and will check the gear color this weekend.

 

I came up with 28.8" for that tire height, verify that this weekend when your working on it.

 

Put a speed app on your phone if you have a smart phone.
I use one to check speeds on various things.
I need to find one that reads in tenths though for some of my uses

 

if your goal is to calculate fuel mileage, the concentrate on making the odometer read correctly, or finding what the error is, so you can make accurate distance measurements.

Those flashing mph signs are hit or miss, as far as accuracy.

 

At what speed is the speedometer 9 mph out?

 

Speedometer was reading 30 and the sign said 39,I know those are not that accurate but should get me closer then what I am now.

 

If you have the 9 tooth(red) my chart show a driven gear with 22 teeth. The 7 tooth( purple) calls for a 17 tooth driven gear. Driven gear is the one that you can remove from the side of the tail housing. I could not find a 17 tooth driven gear, actually the formula called for 17.3 But, there is an 18 tooth driven gear in a brown color. The 22 tooth driven gear is gray in color.

 

Wow, 30 mph reading 39 is a 30% low reading. If the sign were accurate, then you need a driven gear with 30% fewer teeth, a drive gear with 30% more, or a combination whose product is 30% lower.

But I think I would follow Squirrel's advice and calibrate it based on the odometer.

 

There are very limited choices with drive gears on the output shaft, quite a few options on the driven gear.

 

First thing, take off the gear box that is on the speedometer drive and hook the cable up directly.

Those are typically a 1:0.8241 reducer.

39 x 0.8241 = 32.1399

WAY easier to correct from there.

 

Gears and tech can be found here: http://www.thegearbox.org/catalog/item/3864782/3697857.htm

 

The place that I just posted a link to has both a 17 and a 22.

 

I had that site ear marked too, it showed all gears but the 17 tooth. Thanks for posting that.

 

I took the box off and its reading a little faster now,when removing the box I seen it has a red gear and according to that chart it is a 21 tooth gear. Took a drive to that your speed is sign and it was not working which I think is solar powered so I will go by after I get back from the pull and pay tomorrow.

 

My formula comes up with 22.3 for driven teeth, since that can't happen, the 22 tooth I would think would be closest. Assuming your rear 235 tire is 28.8" in height. 22 tooth driven gear should be gray. There are two 22 tooth driven gears, make sure the outside diameter is measured with a dial caliper to match size. Should be .810 or .870.

 

What color is the driven gear that you could remove? Black or yellow? If so, it has a diameter of .870 so, you should use the 22 tooth of that same diameter which is green in color, not gray like I mentioned earlier. Confused?

 

It looked to have a red driven gear when I removed the reducer box.

 

But what color was the plastic gear on the inside of the trans? Not the one that you could remove.

 

Need to start with this gear first, it will determine which small gear to use.

 

.I have not gotten that far yet.

 

When you have the driven gear and its plug removed, just shine a mag light inside you will see the big gear. In what I stated earlier, there were two colors for GM purple(might look like blue) , and red.

 

I had the front of the truck up in the air so I might have had a bath in trans fluid so I did not remove the driven gear,if I can get the rear of the truck up and still be able to get under it I will look.