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Good Day All

Have a '66 GTO 389 Tri-power 4 speed 10 bolt posi with 3:55 gears.
Can this gear set be swapped out to something like a 3:08 without a lot of bother?
Just looking to swap the gears.

The goal is to cut down the cruising R.P.M. to the mid 2K range instead of the low 3K where it runs now.

Thanks to all who respond.

Omni:cheers
 

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Pontiac 8.2 carriers were split into:

High Ratio:2.56 & 2.78
Mid ratio: 2.93, 3.08, & 3.23
Low ratio: 3.36, 3.55, 3.73 (aftermarket), 3.90, 4.10, 4.33...4.56 (aftermarket through 5.57's).

The most common style PONTIAC 8.2 10 bolt ring & pinion sets were used by Pontiac from '64-69. It will have 3/8" RH thread ring gear bolts. The aftermarket gear sets are made with same 3/8" RH threads. Pontiac in late '69 model production (mainly late '69 Birds) went to a 7/16" LH thread ring gear bolt. This necessitated different part number Pontiac 8.2 gear sets along with new single track & Safe-T-Track carriers with 1/16" larger diam ring gear bolt holes.

Have you thought of going with 3.36's and taller rear tires? That would allow you to continue to use the same differential carrier. The other differential solution would be to source a mid ratio carrier & either 3.23's or 3.08's, then have the gears professionally set-up. Personally, I would not suggest going as high a gear as a 3.08 with an M20 Muncie,
 
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