Star79

The 79 I just bought has a 4 sp auto in it, but Bubba didn't change the speedo gear in the tranny. So, it reads about 20% high. I understand GM sells those little gears for under 15 bucks. I just got done cutting my rusty floors out (damn rust!!) and I can get to the cable easily now. Is that all that needs to be changed, or are there actually two parts (gear in trans, gear on cable end)? Anyone have either a part number, or a tooth-count, or pictures, of the right size gear to get me closer to proper speed?

Not sure exactly which 4 speed it is, I can get a picture of it if I have to.


[Modified by Star79, 4:02 PM 5/23/2004]

GasketDude

Post your tire type/size (225/60/15), diff. gear ratio (3.08), and count the number of teeth on both Speedo gears. Drive and Driven. With this info we should be able to get you dialed in.

Pete76Shark

This may sound like "bubba" but I tried to figure which gear I needed and I still came up wrong. So, the local Police were nice enough to setup one of those "your speed is" radar things and you guessed it.... I kept adding teeth until I got it right on the money... Took me 3 times to get the right gear. :banghead:

Schmucker

Find out if you have a 200-4R or a 700-R4

Star79

Okay, I now have some of the answers to the above questions.

Tire size is 255/60-15. Final drive ratio is 3.55:1. The big gear (on the cable) has 40 teeth on it. Here is a picture of it:



Oh crap I forgot to count the teeth on the little (in the trans) gear. Is that one even changeble?

As to whether it's a 200-4R or a 700-4R, I don't know, how do you tell? If it means anything it has a manual electric torque converter switch. There are a couple pictures here if it is identifiable by looking at it:

http://<a href="http://pages.infinit...trans1.jpg</a>
http://<a href="http://pages.infinit...trans2.jpg</a>
http://<a href="http://pages.infinit...trans3.jpg</a>

Like I said, the speedo is about 20% off, can it possibly be as simple as adding 20% to the number of teeth on the big gear?

Again, I'm hoping for GM part numbers too if possible. I'm in Canada and the dealers are pretty useless up here without parts numbers, especially on a 25 year old car.

Thanks again everyone!

-steve

p.s. stay tuned for fun adventures of me trying to get floors from a 1981 into a 1979........


[Modified by Star79, 4:02 PM 5/23/2004]

GasketDude

A 700R4 has 16 pan bolts, 350C has 13 bolts, and a 200-4r has a totally different shape to its pan altogether. Can't tell from your pics if the front corner is "cut" on a 45° angle or not. If so it's a 350C, 3 speed, with 13 pan bolts and a lock-up converter. The 700R4 is totally square with 16 bolts and is a true 4 speed.

Regardless, you still need to count the Drive gear tooth number, betting it's a 15 tooth. Not at my work computer today so I can't give you GM#'s but repost with your Drive gear count and I'll repost tomorrow (Monday).

SpyderD

If you have a 700r4 - this should tell you what you need:
http://www.transmissioncenter.net/sp...n_______va.htm


[Modified by SpyderD, 8:16 PM 5/23/2004]

cw823

I have a brown speedo gear (found it in the battery compartment).....but mine is off about 20mph the other way (reads 50mph @ about 70). I'll have to get info and figure it out.

cw823

oh...and there are websites with the speedo gears for around $9.

Star79

So it's a 700 for sure, definitely a 4 speed and 16 bolts totally square. Also the drive housing is the same as the one on the Transmission Center site.

(that page rules, thanks a lot for that)

The drive gear is blue, so according to the site that would be an 18-tooth one. No doubt changing that one to a 15 would require removing the rear of the trans to get access to the output shaft?

I guess the biggest driven gear I could use would be the 45 tooth (light blue one).

Maybe the answer would be the underdrive adaptor.

Are those the actual GM part numbers on that page?

Thought please experts.

-steve

GasketDude

Seems like changing the drive gear to a 15 teeth would get you dialed in. You already have the 40 teeth gear that goes with it. Was checking on a GM# but my chart isn't quite up to date with all the 700R4 gears yet. ('82 Vettes used a 19t yellow gear.) For a few dollars more you could order it (15t) from the site above. Not sure how you got a 700R4 in your vette, original(?). All manuals I have don't list a 700R4 until '82. Being your going through the hassle of cutting your floors out, replacing this gear on the output shaft shouldn't be to diffucult for you. Those gear ratio adapters can get a little pricey, we use to use these on our 4x4's with 35"+ mudder tires to correct the speedo. Plus they stick out from the trans a bit so you'll have to watch out for floor clearance.

If I was doing this job, I'd change the drive gear to a 15 tooth. This would give me more options down the road if tire/wheel upgrades came about.

Have fun.