So...I have always thought that the best rear gearing for a TPI, automatic car was either the 3.23 gear or the 3.27 gear....depending on if it had the 9 bolt or 10 bolt rear. I was always under the assumption that the 3.42 gears were only used in the 305 TPI, 5 speed cars.

Then I run across an 85 IROC-Z, LB9, auto, G80, GU6, and J65. So this automatic LB9 1985 IROC has 3.42 gears from the factory? Was this only an 85 thing? Or have I just been simply wrong this entire time?

All 1985 LB9 IROCs I've ever seen have 3.42 gears. The '85 LB9 Z28 came with 3.23s.

So it's only an 85 thing??

Yes.

Well I'll be damned....

Thanks Charlie! Of all the third gens I've owned, I have never had an 85, and I had never heard of this. Ya learn something new every day, right? Lol

this

Never knew that. Thanks for the info.

ive owned 4 85 irocs and 3 had 3.42's and 1 had 3.73's .. all 3 of my 86 irocs had 3.23's .. out of the hundreds of 85 irocs ive seen over the years canr recall any not having 3.42's

Cool. I can't believe I had never heard that before.

So your 85 with the 3.73's had to have been an HO car, obviously, and I assume a 5 speed? Or were the 3.73's also available in an HO, auto as well?

yes the 3.73 car was a l69 5 speed iroc .. the 85 l69 5 speed trans am i had also had 3.73's ... seems the 3.73 l69 cars all got g80 posi and a small percentage of the 85 tpi cars had the g80 posi .. heard there was a shortage at the time or something ..

also no l69 auto irocs

Yup, posi shortage after about March. My May built, yellow '85 is a one wheel peel.

Also, the '85 LB9 is slightly unique in that it is an L-69 with TPI, even down to it's L-69 4 bolt flange cat.

so do you think the 85 l69 cars were early builds since all the ones ive seen have the g80 posi ?

Hey, that's a good question.

i just looked one up and it was completed and shipped in feb. of 85 so i bet they were early 85 production

Go to this link on the forum and there is some more information..

https://www.thirdgen.org/forums/hist...2-mystery.html

Weird, I didn't know GM built anything with a 700R4 and gears shorter than 3.23...other than trucks.

That's why the '85s are quite a bit faster than other auto LB9s. L-69 cam and 3.42 gears.

If you get rid of the neutered L03 style exhaust on the 91-92 LB9 cars you do get the 350 cam though.. Hence the 230hp rating on the bigger exhaust 5spd cars though right..

The V6 F bodies with the 700R4 all had 3.42 gears .

Good info here guys. Make this a sticky. I’d bet most guys don’t this stuff.

My L69 IROC does not have the G80 option

Do you know the build date?

Sorry, no, I have no idea....

Look on your driver's side door jam.

I have a sticker, but it is smeared with something and I cannot read the production date.... I have tried everything I can think of to get the smear off, and it won't budge.