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Transmission and gear ratios


Canuck

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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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  • 1 year later...

Hey Aaron.

Sorry that the 70 trans is giving up the ghost. 

I still feel what was suggested above and what you said is the way to go. 

Since the 70 is a one legger, non posi, may I suggest the posi, gear, bearing combo on eBay. 

I myself just purchased this combo and had it installed. I got the kit with a trutrac and 3.31s but it is also available with the Eaton style posi and any ratio you like 

The "guy" that did the work, a friend love known since I was a wee lad asked where I got the parts. I thought maybe something was bad. He said that they were quality parts and the gears set up real easy with a great pattern. He owns a chassis shop and has created some of the fastest and quickest cars in the north east. 

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Aaron, what you are planning has a been done hundreds of times with varying results. 

Lowering cruise RPM only works well if the engine makes the low end torque to support it. I can't tell you the amount of times a guy shows up at my shop with a big inch engine and an overdrive  complaining the car lugs at 75 MPH @ 1900 RPM.  Not an easy task when the build is fairly  new. Every car has a sweet spot and that spot is sometimes not that east to find. 

 

If you like how the 70 cruises, don't mess with success. Just build the trans and be done. On the 71, you could possibly throw a set of taller tires or just regear it to 3.31s. 

 

My 71 is a hoot with a 496, Crazey Daveys old 4500 stall Coan, and 3.73s. But I would hate to daily the car. Already stressing the stock rebuilt ac compressor with alot of RPM. But sideways at 45 MPH is a alot of fun. 

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3 hours ago, Andy's Auto said:

Aaron, what you are planning has a been done hundreds of times with varying results. 

Lowering cruise RPM only works well if the engine makes the low end torque to support it. I can't tell you the amount of times a guy shows up at my shop with a big inch engine and an overdrive  complaining the car lugs at 75 MPH @ 1900 RPM.  Not an easy task when the build is fairly  new. Every car has a sweet spot and that spot is sometimes not that east to find. 

 

If you like how the 70 cruises, don't mess with success. Just build the trans and be done. On the 71, you could possibly throw a set of taller tires or just regear it to 3.31s. 

 

My 71 is a hoot with a 496, Crazey Daveys old 4500 stall Coan, and 3.73s. But I would hate to daily the car. Already stressing the stock rebuilt ac compressor with alot of RPM. But sideways at 45 MPH is a alot of fun. 

Thanks for that info. If I simply rebuild the stock TH350 it would certainly be a lot cheaper. As you say, it is already proven to work well and my gas mileage is not that poor. I am usually very cautious about modifying anything as I worry about compatibility/reliability issues and further mods needed down the road to make it all work right. I’m pretty frugal when it comes to my cars and don’t want to continually throw money at it. For some reason I got hung up on thinking I need overdrive that I stopped considering simply a stock rebuild. There is certainly an argument for keeping it stock.

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Also keep in mind that the anemic 180 hp 305s in the 84-88 Monte Carlo Ss's had the 200r4 and 3.73s in them from the factory. And they cruise down the highway with no problems.

Same for the olds 442s and Hurst olds- 200r4 and 373s. The GM had 200r4 and 3.42s.

Not the 83 MC SS. They had th350s and 3.42 gears. 

Just some food for thought. 

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Remember, those G bodies ran 215/60/15s. 26 inch tall tires made a difference. I had an 87 Monte SS with 3.73s and it cruised all day at 85 MPH at 2200.  Scary part is that 305HO probably made  more torque at 2000 rpm than a big inch built big block.

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What it may come down to is cost/benefit for the 70 Monte with the 350 engine. A majority of the benefit with the OD trans and 3.73 will be the jump off the line with a minor benefit of slightly lower RPM on the highway. I’ll get some quotes and see where things end up. No matter what I do want to put 3.31s in the 71 SS eventually.

I appreciate the advice thus far. :)

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700r4 with 3:73 richmond posi good combo but pulling the Monte Pull Behind to Carlisle we only got about 13MPH.  The 350 was bored 20 over with a roller cam and roller tip rickers

255zr50 nitto 555 tires

 

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200r4, 3.73 Richmond gears and an Auburn posi, Edelbrock aluminum heads, .60 over bore SBC and I got close to 20 mpg rolling to the EM. I have no idea what my rpm's are at that speed but I will say the car is plenty quick off the line if you care about that kind of thing.  

225/70/15 BFG's if that makes a difference.

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I currently get about 19 mpg on the highway with the current setup. All stock 300 hp 350 with 4-barrel AND still running points in the distributor!

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  • 1 month later...

You currently get 19? You don't have much room to complain about that. I can't imagine it getting much better with changes...whatever they may be.

If you are going for MPG, it sounds like you have the perfect setup. Maybe you can set up the OD trans so the extra gear is your first gear..and when the trans gets to OD, it lands exactly where your top gear is now. That would require a lower rear gear ratio.. and your cruise RPMs would be just as they are now. You would be guaranteed the same MPG you currently get, but you would have 3 gears below that for better acceleration.

The same theory is used with a 3 speed vs 4 speed manual trans. Both transmissions have a 1:1 final top gear ratio, so the cruise rpm is the same. The added gear in the 4 spd resides below that to aid acceleration.

I'm just throwing ideas around.

My 454 gets 10 mpg. Uphill or downhill. 

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  • 1 year later...

Update

The 70 transmission seems to be doing fine which is good for now.

My 71 SS454 still has a 3.73 posi rear end. I drive a fair amount of highways where I live and like to take it on a trip once in a while. 3,000 rpm at 60mph is not ideal. I don’t have the budget to put in an overdrive transmission/gear vendor and really trying to keep it as factory correct as I can. I’m torn with just leaving it as is or put in the 3.07 gears I recently acquired. I did manage to track down the original 3.31 gears from my car that are still in the possession of a prior owner in Ohio. At this point I’m not sure if I will be able to get my hands on the 3.31 gears or not. I typically do things on a fairly low budget so it is tempting to use the 3.07 gears that I have but maybe the 3.31 would be a better compromise between acceleration and fuel economy. Anyone else here have 3.07 or 3.08 gears in their big block Monte? Any other comments on this?

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My daughter's 454 El Camino has 308 gearing with an overdrive tranny… Not sure which one … She loves it … 

My Son in law has a 72 Chevelle , 383 stroker ,700r4 ..308 gearing.. He loves it.. 

My Monte has 256 gearing , 700r4 …. I'm a big fan of using the torgue rather than driving down the highway sucking down gas in the horsepower range .. at 3000 plus rpm …. If I need the horsepower it's there and the 700r4 will take you there … 

Ed C… 

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Ok, Aaron said, no overdrive. Anyone with NO overdrive and 3.07s? 

I have 3.31s and th400 in "Big Red" haven't done much highway driving with her yet but definitely better than 3.73s on the highway with a 3 speed trans. I also have 275/60r15s on the rear which cuts the 3.31s down a little further. 

Also "Barn Find" has a th400 and 3.31s with 245/60r15s on the rear. Haven't had her on the highway as yet, hope to change that soon but have been 50mph on the roads and seems ok. Will post back when the weather finally breaks and I get them both out of hibernation.

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2 hours ago, Dtret said:

If you want the ones from Ohio, make the deal and if you need them picked up let me know. 

Thank you! I appreciate the offer but he’s in Streetsboro which is almost 4 hours from you.

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34 minutes ago, Canuck said:

Thank you! I appreciate the offer but he’s in Streetsboro which is almost 4 hours from you.

So what??  If you need I’ll make it happen. Getting it to you would be a little harder. Lol. 

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46 minutes ago, Dtret said:

My 71 SS. 454–th400—3:31 gears w/ posi. Look at the speed and tach. 

4F04D7EA-12F2-4AD2-8E8A-0BAF849487DB.jpeg

That looks good to me.

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52 minutes ago, Dtret said:

So what??  If you need I’ll make it happen. Getting it to you would be a little harder. Lol. 

You are making me feel guilty for any dancing bananas I’ve posted…

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8 hours ago, Canuck said:

You are making me feel guilty for any dancing bananas I’ve posted…

:yay::yay::yay:  No worries. And yes these stupid bananas go against any morals I may of had at one time. Lol. 

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14 hours ago, Dtret said:

So what??  If you need I’ll make it happen. Getting it to you would be a little harder. Lol. 

 

14 hours ago, Dtret said:

My 71 SS. 454–th400—3:31 gears w/ posi. Look at the speed and tach. 

4F04D7EA-12F2-4AD2-8E8A-0BAF849487DB.jpeg

That looks like the numbers on my 72 with a 799r4 and 3:73 gears

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