hookem horns

Previous owner swapped jetaway for TH350 and put in 3.42 rear end on my 69 cutlass. Nice upgrade but speedo is off by 25% (reads 75 when GPS says 60). Weather is cooling off so I decided to look a little closer tonight. Started off badly with a rookie mistake - put front end on jackstands to give me room to work but that meant trans fluid came pouring out when the housing was removed. Dohhh! Anyway speedo gear is in the small bullet housing (chevy?) and the driven gear was white/natural so I'm guessing 19 teeth based on web search. Did not figure out what the drive gear was because of the fluid coming out but assume it would need to change to make up the 25% error. Rather than go through the hassle of that, has anyone just used a ratio adapter like the one on this site - http://www.transmissioncenter.net/sp..._____va.htm#gm ratio adapter

66luvr

Yes, I have one on the 66. I went from a Jetaway to a turbo 400 with switch pitch and a Chevy 12 bolt with 355 gear. I thought about what to do, 400 speedo stuff is not as plentiful as 350 and standard trans stuff. A friend told me about the adapter you are talking about. I knew they were on Chevy 1 ton trucks which use turbo 400s in the 70s and 80s (I've had a few of those trucks and saw them on the speedo cable at the trans) So I did the math as you have, went to my Chevy dealer parts dpt and we looked them up. He said two were available and gave me the ratios, I did more math, and one was correct/what I needed. I put it on a few years ago and the speedo is now very close. I haven't checked it with a GPS but it is very close. It worked great for me.
Good luck,
Mike

garys 68

I'm using one of theirs on my muncie. Works great.

hookem horns

Thanks, think I'll get one for an easy fix.

Redog

I had one on my old Delta which had a 200r4 tranny and a swap from 2.73 to 3.42.

Since you have a 350, why don't you just get the actual little gear instead

oldcutlass

X2- just a thought!

BosMobile

Hey,I just put 3.42 in my sons 83' olds.Speedo is way off!I,d like to change the "little " gear.Does GM still sell the parts?I waited for more on this thread,no luck.So thought id asked,Thanks,BO

Boldsmobile

It is not all that difficult to change the drive and driven gear to correct for a gear change. The adapters do not come cheap and the drive and driven gears are cheap. Many online sources for these. Even your local tranny shop could probably sell you some.

Do you have guys have factory service manuals so that you can look up drive/driven gear values?

I went from a 2.73 peg leg to a 3.90 posi and was able to dial my speedo within 1 mph @ ~45mph and it cost me under $10.

hookem horns

Mine is too far off to just change the driven gear which is real easy. Drive gear & driven gear around $20, adapter is around $60. High for what it is but I don't have to drop the drive shaft so I'm trading a little time for money. Supposed to arrive next week and I'll update once installed.

BosMobile

Yeah,Thanks!I'll research it some.I've got to many projects going right now,anyhow.Thanks again,BO

hookem horns

Took forever to ship, but finally got the ratio adapter. Works like a charm and now reading about 2-3mph slow per GPS instead of 15-20mph fast at highway speeds. Cost more than changing both drive/driven gears but only took 10 minutes and barely got my hands dirty.

Redog

It depends.

If it's a 200R4 it's easier and a hell of a lot cheaper to just get an adapter from the website in the first post.

I went to 3.42's in my old Delta as well, from 2.73, I think it was a 27% slow down adpater needed