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performance and freeway driving
Those terms are mutually exclusive. You really can't have both with a TH350 tranny.

Your TH350 has a 1:1 3rd gear. A set of 2.41 gears will be good for highway driving. A set of 3.73 gears would be good for performance. I had a set of 3.08 gears in my 79 Elky with a TH400 and it had plenty of get-up-and-go and also got about 15 MPG. What you really need is an overdrive transmission and a set of 3.73 gears.
 

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Assuming you have a tire height 26", your engine would be turning 2431 RPM at 55 MPH and 2873 RPM at 65 MPH with the 1:1 final gear of the TH350.

An OD tranny like the 200-4R with a .67:1 final gear would be 1638 RPM at 55MPH and 1936 RPM at 65 MPH.

Plug your info into this Tire Height and RPM Calculator and you can see for yourself what the RPM differences will be.
 

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You might want to look really close at all the options before you decide if you do go with an OD. Some have really nice ratios and seem better designed, my opinion, for street / strip than others. Depends on your engines power band and some other factors.

In the 60's and early 70's 3.73 was a good rear end for hot rodders. That was behind a 3 or 4 speed Standard or a Turbo glide / power glide (2 speed 3 speed automatics). Actually because of the way an automatic handled back then most would go with a 3 speed standard or 4 speed standard.

Me I will always suggest a 4.10 / 4.11:1 gear set with an overdrive by GM. If you have a 350 and a set of 3.73:1 gears then RPMs at highway are up there and gas mileage is not great further the noise of the engine is multiplied by several times at those RPMs. Switching from the 350 to an OD will sharply lower that level on the highway but does get close to the low end of the power band of most HP engines if you leave that 3.73 in there.

The 3.71:1 gears with a 700R4 in final drive its like having a 2.53:1 set of gears with a 350 THD. With the OD and a set of 4.11:1 gears you are at a ratio of about 3.43:1 in final drive gear. (This is very close to the 3.73:1 350 combo but still better RPM and economy) You are still at a decent RPM level but now your further into the RPM power band and should some sucker challenge you (Camaros, Firebirds etc... Generally with a 3.73:1 and an OD) you're in a much better position to rocket off faster then they can. If you kick down a gear at that time then you really get moving faster.

With anything, there are trade offs. You will loose the ability to go faster then say 120 on the highway where as you could get closer to 140 with a shorter set of gears and with a 2.73:1 you could conceivably get up to 160 - 170 Plus Mph on the highway. In an El Camino that is ridiculous to consider they are just not aerodynamic enough to stay put.

Even though your not willing, right now, to change those transmissions out, this will give a little thought to a future project maybe.

Good Luck.
 

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Discussion Starter #6
Hey guys, thanks for your help Im debating,if i keep the 350 I will prob go with the 3.42's ,If i go 4spd the i will hop it up to a 3.73 set up.I just need to get rid of these stock snail gears for sure..I als plan on installing a GM limited slip unit I got out of a doner ss el camino,it had a 3.42 rear gear but they were shot.. ohhh what to do what to do???

\:D/
 

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I am very pleased with my 3.23s
 

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I run around 3000 at 70. It made a big difference over the 2.73s I had before on take off. My box is a 6a, the difference between it & a 6al is a rev limiter. I hardly ever see the upper end of my RPM range so I felt I did not need it. I am very pleased with the system. I run msd on all my cars. Thanks for the compliment.
 

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Discussion Starter #11
Tommy said:
I run around 3000 at 70. It made a big difference over the 2.73s I had before on take off. My box is a 6a, the difference between it & a 6al is a rev limiter. I hardly ever see the upper end of my RPM range so I felt I did not need it. I am very pleased with the system. I run msd on all my cars. Thanks for the compliment.
Ok thanks,A6a box it is for me!!! I will just get the box since I already have the other MSD stuff. and I will try the 3.23's .
 

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I have a buddy that worked for GM most of his life. He was into racing & even worked with one of the lesser know NASCAR teams. Needles to say, I trust what he has to say. He helped me install my gears & it was so easy for him. He told me that it is a major jump to go from 3.08 to 3.23s. Changing gears up to 3.08 will not make a big difference. Going to 3.23 & up the same thing. That is the point when they change from a series 2 to series 3 carrier. I am not saying there will not be a difference between 3.23s & 3.73s, just not that much of a difference. I hope that makes since.
 

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rear gears

Hey all I went the whole way and put a 700R4 behind a reasonably warmed up 350. Drove around for a while with the stock 241 gear which was ok as long as the torque converter did no lock up below 18 hundred RPM. Added 342 gears in August and I love it seems to be just about right for a daily driver. I can pretty much get away from most at the light but still cruise comfortably at 75 MPH tachs about 2800 RPM. Gas mileage is about 17 to 18 if I behave. The best part is the 700R4 it has the low gears when you need them. Just make sure it is built well and at least came out of an 89 or later GM. Good luck on you selection.

Harald K #219
 

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I used to run a 3:73 and little old ladies in mini vans were passing me as I buzzed along the freeway at 3500 rpm. I switched to a 2:73 and the highway was much better. It's a little slow on pick-up but not bad. I used to have a 3:23 with a four speed manual and it was perfect. My big block chevelle has 3:31's and it's factory.
 
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