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Old 12-10-2010, 02:13 PM
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Whats the gear ratio in a 1972 pontiac lemans

Whats the gear ratio in a 1972 pontiac lemans, not sure what was originally in it for a motor. It now has a 389 out of a 1965 GTO. Dont pick on me cuz I'm dumb!!! Im a girl so I need a little help.....Please
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Old 12-10-2010, 03:38 PM
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Being that its a lemans I'm going to guess that its hi 2's to low 3's(2.75-3.08) or something close. Highway gears definitely. I'm also willing to bet that it came with a 350 2bbl engine. You can check by taking off the differential cover and counting the teeth on the ring and pinion gear or you can jack up the back of the car and put it in nuetral, with chalk mark one of the tires at 12 o'clock position and turn it 1 full turn and look at the pinion where the u joint is and count how many times the pinion turns when 1 full turn of the tire. The tire method won't be exact but will be close if you keep a good eye on where the pinion stops and make a good guess at where it stops. 3/4 of a turn equals .75 for example.
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Old 12-10-2010, 04:51 PM
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This will probably have the corporate 8.5 10 bolt rear end in it. If it has a standard differential then leave one wheel on the ground and turn the other wheel exactly 2 full turns. If the drive shaft turns just past 3 turns then it's a 3.08, if it turns 2 3/4 turns then it's a 2.73, if it turns close to 2 1/2 turns then it's a 2.56... If it has a good posi and both wheels turn together the same direction then turn both wheels exactly one full turn and watch the drive shaft. It helps to have at least 2 people do this.

The best way to do it is to remove the cover and count the teeth. Divide the number on the ring gear by the number on the pinion. 41/15 = 2.73 The oil probably needs to be changed anyway, so this would be best.
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Old 12-10-2010, 05:27 PM
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Thanx Biggearhead for clarifying the posi- no posi difference with the tire method. I think she has the BOP 8.2" 10 bolt diff if i'm not mistaken though.
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Old 12-10-2010, 06:48 PM
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The corporate 8.5 started taking over in the A body cars in '71, except in the Chevelle. There is a very good chance that it has the 8.5.
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Old 12-10-2010, 07:24 PM
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The stats section at ultimategto.com may shed a little light on what it came with from the factory. If i were to guess I would say 2.73 as the EPA was starting to crack the whip.
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