How To Speedo Gear Correction

[Frijolito1988]

Soo I really dont get this. I have the tire size, and rear end gears, so once I figure out what my driven gear is , and I put it in this formula , it will tell me what drive gear I have or what drive gear I need ??

http://www.bgsoflex.com/speedo1.html

the other way around If i only want to change the driven gear, do I put the drive gear I have and it will tell me what driven gear I have or need ?

[Average Joe]

Great info here!

[Sklptrljay]

Nice thread. I am getting ready to do this little project, and I found the TCI calculator simpler and faster to use than the one in the original post.

Here's a link if anyone needs it.

http://www.tciauto.com/tc/speedomete...alculator/#350

[MARKDTN]

Great post. The only thing I might add is that the "bullet" that holds the driven gear can wear over time-especially the aluminum ones-and leak. My recommendation is that if it is aluminum replace it. If it is steel give it a good looking at. If the hole that the driven gear goes into is oblonged at all, replace it. Yours is steel and looked newish. This is only for Chevy TH350s with small driven gears. Won't apply to TH400, 200R4 or 700R4 or any TH350 with a BOP tailhousing (with large driven gear).

[rmawyer]

Thanks for the information. If I had someone else do the work, would a transmission shop or general mechanic shop be the best choice? Also, roughly how much should it cost to have this done by a professional? My speedo is showing roughly twice what I'm doing (100 MPH when I'm doing 50 MPH). Any suggestions for a good shop near Cambridge MD would be helpful.

[snipescastle2]

one note for the guy's running 700R4 trans, if you have to change the speedo gears to correct for rear end gear changes, depending on how low or High of a gear set you run, you will need the correct driven gear housing, for driven gears 34 to 39 teeth or for the lower gear sets, 40 to 45 teeth. in GM's infinite wisdom, they stopped making driven speedo gears from 40 and up teeth, so you are left buying an after market gear. I bought the matching housing and gears off Ebay for about $35.00 since I am running 3.73 rear end gears, I needed a Gray, 15 tooth drive gear for the output shaft, and a Black, 40 tooth driven gear for my setup in order to correct my speedo. now, the only thing I wasn't aware of with regards to installing the drive gear on the output shaft of the 700R4, and I'm not sure if the 350 turbo's have this also, but there are two small dimples that the spring clip that holds the drive gear in place on the output shaft, you have to use the forward one of the two holes, if you don't, you'll get what I just encountered.....the gears will not mesh correctly, and they will grind each other's teeth off!!! the whole idea of having the correct housing for the size of the gears for a 700R4 is because they offset the drive hole to accommodate the larger diameter of the gears, but if you don't get the drive gear aligned to the driven gear, you'll be ordering another set of gears just like I did. Hope I save someone a few bucks by not making the same mistake I did.....here's a picture of the gear housing I mentioned...
Ben