Speedometer gear?

[m3talc0re]

From what I can tell, changing the driving gear is a pita.. I haven't had the chance to look at the gear yet, though. I'm hoping it's not eat up like that driven gear XD

[hatzie]

From what I can tell, changing the driving gear is a pita.. I haven't had the chance to look at the gear yet, though. I'm hoping it's not eat up like that driven gear XD

You have to pull the driveshaft and then pull off the extension housing.  Really not that bad. 
Hopefully it's not chewed to pieces but it wouldn't be shocking.

[Omegadog]

I'm having a similar issue with my truck. It's a '84 305 with a 700R tranny. I have and will continue to use 235 75R15's. My speedo reads 45 when doing 55MPH. I spent some time trying to get the right color coded gear but can't seem to figure it out. I will look for the gear ratio next time I can get under the truck.  Also, I think that my tranny is a bit slow to upshift so does that mean I need to replace that gear as well? Thanks for any input and Happy New Year!

[hatzie]

The 1984 700R4 has FIVE pages with tables of possible Drive gear-Driven Gear-Ratio adapter combinations for axle ratio and tire size combos in the parts manual.
You'll need the axle ratio to go with your tire size.  Once you have that it's not rocket science to read the tables.  As long as nobody has changed the axle ratio in the last 33 years...  The axle ratio RPO may be on the SPID label in the glovebox.  Another option is to feed your VIN to http://www.compnine.com to get the original build equipment list. 

http://gmpartswiki.com/getpage?pageid=59812
http://gmpartswiki.com/getpage?pageid=59825
http://gmpartswiki.com/getpage?pageid=59826
http://gmpartswiki.com/getpage?pageid=59827
http://gmpartswiki.com/getpage?pageid=59828

The TH700R4 uses the 2" diameter driven gear adapter that has to match the tooth count of the driven gear. The BOP TH350, TH400, & NP208/NP241 use this same setup.

This is the Chevy TH350 with the 7/8" bullet driven gear adapter.

[Omegadog]

Thanks hatzie. I'll follow through and report back. Very helpful videos. I need to replace my rear seal on the housing anyways. I know that the 305 is not original to the engine but I'm not sure about the tranny. The SPID is long gone and there's a new glovebox in her. I'll look up the VIN to see about that and if the gear ratio is the same, I suppose it may be the original tranny. You folks are very helpful and knowledgeable here. I think that I'll be hanging here a while.

[ehjorten]

This is super-easy to do!  Pull the driveshaft and then pull the tailshaft extension housing.  While you are in there, get a new o-ring to reseal the extension housing and you will want to replace the tailshaft seal and possibly the slip-yoke bushing too while you are in there.

[82silverado]

http://www.laspeedometergear.com/

I went through all this with my 82 with a 700R. The link above is where I got my parts.

http://www.tciauto.com/tc/speedometer-gear-calculator/

This link is a page to calculate what you might need for a GM application.

[m3talc0re]

I didn't pull the housing off, I just used a mirror and light to look into the hole. The drive gear is green and  seems okay. I could barely get my finger in there to feel it, but it didn't look or feel chewed up.

[m3talc0re]

Well, I got my stuff in and... it's not the right combo XD I got the red gear. Speedo is off. It says I'm doing 50 and actually doing 60. 58 for 70. It's also a little bouncy. There was minimal play in the housing, but I guess it's enough. I didn't change the driven gear, it seems okay.

hatzie, I forgot all about your blue gear suggestion :/ Guess I'll need to order it, lol.

[m3talc0re]

Well, I got the gear replaced. The drive gear was fine, changed it anyway. I put the blue driven gear on there, but the speedometer reads 10mph slow (going 60 says 50).