New speedo gears needed
 

I have a stock TH350 trans and upgraded the rear to a 3.73 ratio. Anyone done this before and know what size driven gear to use so that my speedometer reads correctly. Just checking if someone has already done this before I go through all the equations and the math.

There's no right answer for that. You have to know the drive gear teeth too, as well as tire height.

The drive gear could be any number of teeth, so you must know which one you're running to figure which driven gear you need.

www.tciauto.com/tech_info/speedo_gears.htm to convert tire size to diameter
www.novaresource.org/speedo.htm uses gear ratio & tire size to find speedo gears
www.novaresource.org/speedo2.htm 2nd page of above

Gears are available from the Chevy place, TCI, & possibly board vendors.

Jim the last 2 links you provided don't work. Anyway I got underneath my truck today and was able to get the speedo cable out of the trans tail housing and the gear that came out with it is a 19 tooth what I believe is the driven gear. I still dont know what the drive gear count is because I could not find it. I know it is on the output shaft but do I need to drop the driveshaft and unbolt the tail part of the housing? or now that I know what size driven gear I actually have do I just need to get a bigger size gear to slow my speedometer down to actual speed I am going. I saw that on TCI's website that for a TH350 there are 20,21,and 22 teeth gears which I think is what I need. The next biggest one is 34 teeth and I think that will slow down my speedometer too much.

http://members.core.com/~faldrich/speedo.htm
http://www.sccoa.com/faq/speedgr.html

Speedometer Correction
 

http://www.bgsoflex.com/auto.html
Check out this link, you'll like the other auto-math calculators too. TX is right, you have to count the drive gear teeth. You may be abe to see it with a inspection mirror, if not you'll have to pull the tail housing. What was your original rear gear? Looks like you can calculate drive gear on this site as long as you know driven gear size and orig. gear ratio.

go to your local GM dealer. they will have a chart in the book saying which gears to use for what tire & diff. look for the guy who has been there for a while as he will know where to look.
I have done this many times, they always seem to have the parts in stock or able to get them in a day or 2.

Thanks for all your help guys that is what I needed for the links. I don't know what the original rear gear ratio was so that won't work. I guess next chance I get I will pull the tail housing. I know I am going to have to remove the drive shaft, but what do I need to do for the tail shaft. I know there are 4 bolts that bolt to the trans housing. Any thing other than that? Any problems I should lookout for? Is it a problem putting it back together to get all the gears to line up right?

Here's an easier way. Get on the highway with a friend and a good watch with a second hand. While you drive at a steady 60 mph, have your friend check your speed between the mileposts with the watch. 60 mph is one mile in one minute. Then adjust your speed until you are covering one mile in 60 seconds and note the speedometer reading.

Then take your speedo gear to either a dealership or a tranny parts house, tell them that the gear in your hand gives you a reading of XX mph when you're actually going 60. They should be able to do the math and sell you the correct gear.

What was your old rear end ratio? Divide 3.73 by the old ratio & multiply it by 19. This will get you close, unless it comes out more than 22. If novaresearch doesn't come back up, I'll see if I can find the sheet where I printed it out. At least I think I printed it out.

NEVER MIND, I HAD A BRAIN F@RT, & THE URL'S HAVE BEEN CORRECTED
www.novaresource.org/speedo.htm
www.novaresource.org/speedo2.htm

Even easier (if one is available) is to use a GPS or find someone that has one. I'd help you out, but I'm a little too far away. :D

When looking for replacement drive or driven gears National Parts Depot has more of a selection than TCI. GM parts department could not help me when I asked for a driven gear with a certain number of teeth. They go by part number only. I ordered mine through NPD worked great.

Even better. Lots of great info here . I didn't even think of using a GPS system and jim I divided (3.73/3.08)*19 and came up with a 23 tooth gear.

Speedo gears are less than 10 bucks a pop, and about a 5 minute swap. I'd give that 23 tooth gear a shot.

I found that out today when I went to the parts store. So I bought 3 of them. 21, 22, and 23 count. Use the one I need take the other 2 back.