I am going to put new gears and a posi unit in my 69 firebird,wondering what some of you would recommend.here`s the info,350 engine bored .030, edelbrock 4 barrel carb and performer intake,hedman headers,hei distributor,true dual exhaust,350 tranny with shift kit ,8.5 rear end,28 spline axles.thanks allen

Are you looking for more of a driver or street killer? A daily type driver would be fine with a 3:23, which would use a different carrier. I would suggest you go with the carrier that would allow you to go steeper with the gear if you wanted to bump up.

This one on Ebay the carrier for a person that might want to play with the gears for better hook-up. I am not telling you to buy this one, just showing which carrier I would use. As for ratio with the below carrier, 3:36, at the most 3:55.

http://cgi.ebay.com/ebaymotors/GM-BOP-BU...=item1c06fd3799

have the 8.5 corporate 10 bolt, with a 3.42 posi and like it alot. Made a huge difference from the 2.78 open I had prior. I am running a 400 HO, but the engine was only at 8.2:1 CR. Still had plenty of pop when I wanted it. I am now building a stock 400 HO plus a little, and it should be even better.

thinking about 3.42 gears or 3.23 gears, but wondering what r.p.m.`s i would be running at 70-75 m.p.h.,anybody got any info for me on that?not a daily driver. what i would like is that kick in the seat of your pants feeling at takeoff and an rpm range i can live with on the highway on a cruise.

I had a 3:36 gear that gave me a lot of get-up and go and did not run too high on the rpm's?

This question come up a lot but the real question is what is acceptable to you for a 60mph cruising RPM and what is acceptable for off the line acceleration. For my self I like the reliability of a th400 so a 4sp auto is not an option to me. I have had 3:36, 3:23 and 3:42 in my two pontiacs and now with my advancing old age I'm gathering parts to put in a 3:08 gear. Keep in mind the 455 has lot more torq and doesn't need the steep gear as bad as a 400 or 350. In your case I would lean to the 3:23. I had them in my 77GP w/400 and it was a good compromise.

From what I recall at 60 mph and depending on tire size: (maybe some other can fill in here too)

3:08 = not sure
3:23 = 2500rpm
3:36 = 2700rpm
3:42 = 3000rpm
3:73 = 3500rpm

My $.02

IMO 3.55 gears are the best of both worlds. PLENTY of get up and go off the line but still drivable at higher speeds. I wouldn't go with anything other than 3.55 or 3.36 EVER in ANY Pontiac...

I have a 3.36 gear in my '68 400. Still breaking everything in.

3:36 is a great gear for both automatic and manual street-able cars.

I have a 3.55 in the 67 with a Th400 and love it.

IMO, depends on how hot the engine is going to be built. If you have a low performance, small cammed 350 with 2 bbl heads, you wont have use for anything steeper than a 3.08 or 3.23. You wont have the torque.

Now if you replicated the 350 HO with #18 or #48 heads with 65cc's and an 067 or 068 regrind and decent compression and headers or longbranch manifolds, I would go into the 3.36 or 3.42.

The 350 has the same stroke as a 400 and likes to be wound out as quick as possible. If you don't have the grunt in the low end, doesn't matter what gear you have, it will fall flat on it's face.

I had a 400 with a Torker II intake, 3.55 gears, 9-1 compression with a 245/ .510 Lunati single profile and a 3500 stall. It had nothing off the line in first and shifted at 4500. Once I got the gear up to 3500 RPM it would freight train in 2/3. I changed the heads to #13's and the [censored] thing screamed through all gears and had a traction problems even with 275/60/15 rears. The engine got even better changing to a stock intake. So..I guess my point is...You need compression and good flowing heads for a decent gear to work under any circumstance.

i have #47 heads from butler performance that are #48 head replicas and an 068 grind cam.i have an 8.5 differential,i don`t think they make 3.36 gears for the 8.5 thanks allen

Sorry Allen, all of my information was based on a 8.2 rear. I would go with the 3:42 if you are running the 8.5 rear.

If the compression is up around a well tuned 10 to 1, go for the steeper gear.

Just chiming in again. My 3.42 gear set up includes 4 speed M22, and what was a tired 8.2:1 CR engine 400HO with 066 cam grind. the car could always break tires loose, and get sideways for as long as I stayed on the throttle. Rebuilt motor will be 9.7:1 with the Summit #2802 (close to a 068 i'd say), most else stock components, my builder who did the machine work thought that should be a pretty good set up for my uses.. for what its worth.

At 70 MPH, 3500RPM. I run a 3:55.

Here's a handy calculator for those who want exact numbers.

http://www.csgnetwork.com/multirpmcalc.html

I have had 4.11's and you could barely shift fast enough to keep up with them. Made driving around town like driving a tractor!

3.55's now and love them. I have a Keisler 5 speed so the first 4 gears are still steep and put a smile on your face everytime but the OD on the highway drops right down 2200 rpm @ 70

If you have a 8.5 I would go for the 3.42 as a best of both worlds gear and steeper if you are heavier footed!