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I have a 70 Gto convertible that has the 455 HO and the turbo 400 with a 3:07 non possi rearend. I really want to change out to a possi unit but not sure what I should change too. I want to keep it an original part and
The correct possi maybe 3:31 or 3:55. I love the 3:07 on the hiway
But off the line it just shreds the poor right tire
 

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I have a 70 Gto convertible that has the 455 HO and the turbo 400 with a 3:07 non possi rearend. I really want to change out to a possi unit but not sure what I should change too. I want to keep it an original part and
The correct possi maybe 3:31 or 3:55. I love the 3:07 on the hiway
But off the line it just shreds the poor right tire
I have a '71 GTO with the original 400 engine, TH400 transmission and a 3.55 posi. I don't believe this car came with that gear ratio or even the posi. I have the PHS sheet so I'll look again. Has factory air. The 3.55 is fine for all around driving. A little street action and maybe a trip to the strip. It doesn't like interstates too much at all. Twice a year I have to use them for only a short distance. I normally drive about 45-50 mph, tops. No rush. This is really a very nice car I have. Body off frame resto (not by me). But, plain and simple truth, the car is a slug. I have to power brake it to spin the rear tires and even then, the turning doesn't last long at all. I can't imagine a lower numeric gear ratio. I have a Voodoo cam, RA exhaust manifolds and sent the original Q-jet out for a professional rebuild. I personally think the carb. is holding it back, but I'm not going to change it out. I know I got off your subject matter some, sorry, but as to just posi units, Auburn and Eaton, both make posi's for Chevys. I'm not sure what they make for Pontiacs, Nicholas.
 

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If I could also add, I have a '62 SS 409/380 that came with a BW T-10 and 3.70 posi rear. It might not be a fair comparison of automation transmission vs. manual, but those 3.70 gears, were way too much for me. It would kick ass from light to light, but highway driving, forget about it. I went to a 3.36 posi and have never been happier. Plenty of power. Lower rpm's when cruising. Engine runs cooler. All beneficial. I also forgot to mention that my '67 GTO came with a 2.56 posi and 4 speed. I don't know who put those in or why, but terrible. I changed them out to 3.55's. OMG. Thought I died and went to heaven. A great gear for that car and my driving habits. Of course, I couldn't leave things alone. I then went to 3.73's. Big, big mistake. I don't think performance was that much better and other things suffered. I gave this car to my son about 4 years ago and he beautifully restored it. He didn't like the gears either. Went to either a 3.31 or a 3.42 set. Much, much better. I guess what it comes down to is what you expect from your car and your individual driving habits. Best wishes with your selection, Nicholas.
 
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