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Transmission and gear ratios

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I am starting to consider plans for both my Montes.

The 1970 Monte (stock 350 4-barrel) currently has a TH350 trans and 2.73 non-posi 12 bolt rear end. The 1970 is now at 122,000 miles so I know a trans rebuild is not too far away. I drive that one roughly 4,000 miles a year to work and back so an upgrade to an overdrive trans is considered. It gets pretty decent gas mileage in its current configuration.

The 1971 Monte SS (stock 454) currently has a TH400 trans and 3.73 posi 12 bolt rear end. The car had 3.31 gears originally. I drive this car to FGMCC meets so the 3.73s aren't the best for the highway use. Ideally simply want to change the rear end back to 3.31 or something close.

I would like advice on the following:
1. I am considering that when the time comes to put a new trans in the 1970 I would upgrade to an overdrive transmission and move the 3.73 gears over to the 1970 from the 1971. Both are 12 bolt rear ends so I assume this isn't too involved? Based on my research the common 200r4 has a 0.67 final gear ratio which would put me at slightly lower RPMs than my current stock configuration. It would not be significant gas savings but would be a lot more fun to drive and still save a little. Woud 3.55s be a better match with the OD trans?

2. After moving the 3.73 over to the 1970 I could put something more ideal for my driving habits in the 1971 Monte. Maybe 3.31 or 3.08 would be a nice all round gear? I'd like to keep this one stock (ie no OD trans or gear vendor).

Any advice is appreciated. Thanks!

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Hey Aaron,

I personally would put a 200r4 with the 3:73 12 bolt with a 28" tall tire in the  70. Like a 275/60 r15. Have had this combo in a few of my rides and liked it. 

For the 71 I would go with the 3:31. Love it in my 70! I feel with the 3:08 you would not like it around town but on the highway you would. The 3:31 in my opinion gives the best of both worlds. 

Either way, good luck! 

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