So I have a 1969 F85 Automatic th350 and I'm looking to get either new rear end gears or pulling a complete rear end off a new car. Which rear end would be a direct fit? Also, which gears would be good to get off the line a little faster, but don't sacrifice mpg and still be able to drive on the highway.
Thanks!

 

The ideal rear end would be out of 71-72 cutlass, parts are readily available. Look on craigslist, I got mine for $200 out of 71.
I would go with 3.42 or 3.23 gears.

 

Need to verify your current ratio first (2.56 / 2.78, etc) if its 2.56 I would say go to 3.08

 

I should have also been more specific, I am also hoping to get a posi if I get a new rear end completely and how would you identify the rear end gears?

 

To be sure of any gear you really need to pull the cover and get the tooth count of ring and pinion, ditto on what he said about the 71-72 Cutlass or Skylark , they have the Corporate 8.5 rearend which gears and Posi units are easier to come by which I just get from newer 1/2 ton 1500 series pick ups or suburbans. Depends on if you find a local deal. I would not turn away from a good used Posi rear from a 68-72 Pontiac (8.2 10 bolt) GTO Lemans either, just as a 68-72 Chevelle 12 bolt 8.878 is the best overall for strength and parts but pricey unless you score a deal. If your stock gears are 2.7 ratio a 3.23 is about as far as I would go to not kill highway economy but it will go down as the ratio increases no way around it unless you go with a overdrive automatic swap like a 700R4 with adapter or a TH2004R direct fit , the 700R4 though it needs a adapter it is much more common and has a deep 3.06 low gear that matched to a new 3.23 or 3.42 posi setup would really get up off the line and the OD would cruise like a 2.56 gear did with the TH350. I would pull your cover off and just do an oil change on yours now just so you know what you got if it was me. A fairly easy source for a 8.2 rear that frequently is a Posi and a good 3.08 or 3.23 gear is from a 69-72 Grand Prix and its a direct fit. Also looks for 70-72 Monte Carlo or El Camino for 12 bolt Chevy although those are usually 2.73

 

Bridgetown Ohio , 70 442 , 3.42 Posi $900
http://cincinnati.craigslist.org/pts/4630955996.html

Dayton,OH , 66 Cutlass 3.08 Posi $800
http://dayton.craigslist.org/pts/4597727858.html



Also worth noting if one builds up a 71-71 Cutlass/Skylark 8.5 housing and wants a upgrade on new Bolt in alloy axles can be ordered from Mosier as a stock replacement but instead of 28 spline you can order 30 spline and that allows you to either use a 30 spline full spool or even a junkyard posi out of a 90's Chev/Buick wagon or sedan caprice, impala SS , roadmaster etc.

 

The rear from any '68-'72 A-Body will fit. This includes:
Buick: Skylark, GS
Chevrolet: Chevelle, Malibu, Monte Carlo, El Camino
Oldsmobile: Cutlass, F-85, 442
Pontiac: LeMans, Tempest, GTO, Gran Prix

To verify the gear ratio, you can pull the cover and look for the stamping on the edge of the ring gear.

The pic below shows what it will look like. The numbers 11 and 41 are the tooth count for the pinion and ring gear. 41 divided by 11 = 3.73 ratio.

IMG_0221.jpg