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Old 03-06-2005, 05:03 PM
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Posi Rear for my 1971 LeMans

New member here, hoping to get some benefit from your collective knowledge.
I have a 71 LeMans with a 350, and 350 TH Trans.
I recently added a dual plane manifold, Holley 600, HEI distributor and a mild hydraulic cam. (1000 - 5500RPM range)
What would be the reccomended tire size and rear end ratio, for use a s a daily driver?
I have an open rear now, and would like to install an aftermarket posi unit, but don't know if I have a B-O-P rear or a Chevy rear.
Any help is appreciated

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Old 03-06-2005, 08:34 PM
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First thing you need to find out is what rear-end yoy have in the car...like a 8.5 10 bolt or what ever. A good rear gear would be a 3.55 or a 3.73 if you want a redlight to redlight racer, that is and still be mildly steetable. As for the posi unit you would be better off to get a detroit locker unit to bolt in.

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Old 03-07-2005, 03:36 PM
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The rear is a 10 bolt open rear (I believe 8.5), but as I said , I don't know how to tell if it is a Chevy or a B-O-P. From the info I have seen on-line the after-market posi units are different. I thought 3.55 would be a good streetable rear, just did not know if I should go higher(3.73, 4.10, etc)
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Old 03-07-2005, 08:32 PM
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You want streetable gears with a TH-350 and are open to tire size.
Shoot for a 26 to 28 inch tire, each 1/2 inch in height equals roughly 1/2 mile per hour speed difference.

Try for a combination that gives you a good range for takeoff and highway, something similar to a 3.42 gear at most and and more toward the 27 to 28 inch tire size, 235/75/15 measures close to 29 inches for reference.

(Section width in mm) / (25.4 mm per inch) X (sidewall height ratio) X (2 the tire on top of, and bottom of the rim) + (15, the mmeasurement of the rim) = the tire diameter size in inches.
235, divided by 25.4, times .75, times 2, plus 15, equals 28.87795 inches.

And remember not all tire manufacturers tires of the same posted sidewall size will measure the same as other manufacturers tires.
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Old 03-08-2005, 05:49 AM
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If your rearend is stock it is either a BOP 8.25" 10-Bolt (not to be confused with the Chevy 8.2") or the corporate 8.5". '71 was the transition year for the 10 bolts in many models of GM. Either one is good enough for a mild street car. 3.55 gears would be just about ideal for you combo, especially if you wanted to slip a 200-4R in place of the TH350 later on. A 3.73 or 4.10 is a "lower" ratio and you do NOT need anything that low for a mild street Pontiac. Even a real healthy Pontiac will work good with a 3.55.

I had a '67 GTO years ago with the 335HP "YS" 400, TH400 and 3.55 8.25" Posi. I had bored the 400 .030" over, added a Cam Dynamics Stage II Hyd. cam, Torker, headers, etc. and had the trans rebuilt with all B & M innards. It would break the tires loose on a full throttle 2-3 shift and I don't mean "chirp" the tires, I mean break 'em loose. The stock 10 bolt held up just fine. That car was my daily driver for 2 years and my "toy" for another 1-1/2 years, I put 35,000 HARD miles on it in that time, many of those 1/4 mile at a time.
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