How to check rear gear ratio
 

I just want to make sure I do this correctly because I remember reading a few different things. I think there is a chart somewhere with rpms vs mph but I couldn't find one on any searches. I also thought that it was possible to count the teeth on the gears to tell what ratio it is.

As for some background, the original owner sold it to me and said he put 3.73 gears in it. When I had the shop change my speedo gear accordingly, it made the speedo even more off. Because I don't know how fast I'm going, the chart with rpms vs mph won't really work for me. I also just had the diff fluid changed, so pulling the cover and counting the gears would be a PITA and would require having that done again. Do I have any other options? Thanks in advance guys...

~Matt

1.) Get the car in the air with your front wheels safely chocked and the transmission in neutral. Parking brake off.
2.) Mark a rear tire in the 12 o'clock position and the quarter panel above it with a piece of tape
3.) Get under the car and mark the driveshaft in the same way; with a mark on the shaft and an orientation mark on the center section (if you can)

Turn the rear wheel one full revolution while counting the number of times the driveshaft spins. If it spins roughly 3.75 times around it will be a 3.73 gear. It helps a lot if you have someone else turn the tire while you count the driveshaft rotations.

Bill, you beat me to it! Your explanation is right on!

This method only works with a clutch or cone type posi. Screw and open diffs are difficult to read.

My factory 9 bolt is 3.27 posi, but I'm not sure if it still works. I'm going to call the shop in a few minutes to see if I can throw it on their lift this afternoon to check it. Thanks again.

im a lil confused, but if you have a 9 bolt, its a good bet that you dont have 3.73. maybe 3.45s

yea, yea lol. well... if the previous owner said 3.73 .... 3.70 is possible yea, but not technically 3.73

Well I definitely have the factory 9 bolt, but the previous owner claimed they were changed to 3.73.

I can't open that link right now because my work server thinks it is porn, but I'm guessing it shows availble gear ratios for a 9 bolt.

I'm taking the GTA to the shop today and he said he would take a look. He mentioned that they did turn the rear wheel and count the driveshaft and that it was 3.73. He also said that they got the speedo gear from a GM dealer with 44 tooth driven which sounds correct. I'm thinking maybe they never changed the stock 17 tooth drive to the required 15 tooth.

Ok so the shop changed the speedo gear in the dash (which didn't need to be changed) and never changed the gear at the tail of the trans. So they're getting that as soon as they can and then hopefully my speedo will work. It is off by almost 30 mph right now.

~Matt

speedo gear in the dash????

yeah, birds dont have speedo gears in the dash, ever, and the dash is electric after 85, so they are full of BS.

the gears are pretty simple to change, you can do it in the driveway.

Yea he said he had replaced the 45 tooth speedo head gear in the dash but not the gear in the tailshaft. The tailshaft gear has to be changed to 15 tooth drive from the stock 17.

Maybe he meant the gear by the speed sensor on the trans cable? Isn't there a gear there too?

When i got my new rear with 3.73s in it, I had a dakota box installed to adjust my speedo.

there are two gears in the trans, one on the shaft and the other is in the same spot just off to the side. they are both in the tailshaft section, i used to have a pair of gears just for that combo.

I can't speak for 3rd gens, but in older stuff there was two ways of doing it. There's a steel "drive" gear pressed on to the tailshaft of the transmission- of which there are different sizes. For each one of these "drive" gears, there's a set of plastic (with one exception) "driven" gears that connect to the speedometer cable. In order to correct your speedometer, you would change your "driven" gear. If you need to change it to a ratio that's not supported by your "drive" gear size, you have to change the "drive" gear first, to allow you to use the "driven" gear that works with your ratio. Clear as mud, I know. If you use the wrong plastic gear with the wrong drive gear it will either chew it or read erratically depending on which gears you use.

In some cases, there are speedometer conversion boxes out there which keep you from having to change the "drive" gear, or to help you get a particular ratio that isn't available.

I am pretty sure that for newer cars with an electronic speedometer, you can just reprogram it. In early electronic-speedo cars, you need a conversion box since reprogramming is not an option? Somebody correct me on this if I'm wrong.

I have no idea what he did because everything you guys have said, and I have researched, has shown that there are two speedo gears and both are in the trans.
A few key things he said yesterday:

-He took out a 45 tooth gear (I told him stock was 44, he claims he thought it was 45. Also the local GM dealer he called said they don't make 44, but I've seen them online).

-He put in a 40 or 41 tooth gear (I don't know why, since thats going the wrong direction)

-When I mentioned that all I needed was a new 15 tooth drive he said, "oh that means I'll have to change the one in the tail of the trans and I'll need to order some new gaskets too."

From this, I would assume he changed the tooth driven (on the VSS of the 700R4) and didn't touch the tooth drive (in the tailshaft). This link shows how to change the two gears:

http://sethirdgen.org/vss.htm

According to this link below(post #10), my stock 9 bolt with a 3.27 gear and stock tires (245 50 R16) required a 44 tooth driven and a 17 tooth drive from the factory. That means all he needed to do was change the tooth drive in the tailshaft to 15 to match the aftermarket 3.70 rear gears that the original owner put in. Now he has to order another 44 tooth driven to put back in and get the 15 tooth drive to place in the trans tailshaft.


http://www.thirdgen.org/techboard/tr...eedometer.html