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Discussion Starter · #1 ·
im changing to a 700r4 from a 350 im changing the gears from a 256 to a 373 or a 342 now with the 342 at 60 miles an hour the rpm is 272285,with the 373 its 296965,[my tires are 25 and a half high [they do not make the 355 gears up here,my original choice]my gears right now with the 350 trans are 256 and the rpm at 60 is 203815 with the 700 in overdrive at 60 with the 342 the rpm is 185627,and with the 373 its 202453,so when you drive the overdrive trannys you only use the overdrive on the highway correct???and you drive around town in drive,so where am i getting any benefit from the 700r4??????if i use the 256 gears with the 700r4 and drive it in drive i get the same rpm 203815 as i have with the 350 tranny and the 256 gears ???????im confused please explain this any help greatly appreciated
 

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If you do not intend on driving your car at highway speed, you will not get such benefit out of an overdrive trans. You can however install a lower gear in the rear to help increase your acceleration from stop or low speeds and run a comfortable cruising rpm if you do any highway driving.
 

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Somewhere on this site is a thread about the final ratio. IT involved imput shaft rpm, tire diameter and rear end ratio which resulted in a final ratio.

It doesn't matter which number you change, its the final ratio that determines what your cruise RPM will be.

IF you lower your imput shaft RPM via, say a overdrive trans, thats one way of doing it.

You can lower your rear end ratio number or change your tire size. etc. Or play with all three, The end result is your final ratio.

Does that make sense?

Here is a chart for figuring out what you need.

http://tri-5.chevyrides.com/rpm.php

An example could be a S10 non world class 5 speed. First gear ratio is something like 4.0 (a crawler gear if you have a 3.73 rear end), but mate it to the 2.56 rear end and you have a ussable first gear and the .86 OD fifth gives you good economy on the highway.
 

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The 700r4 serve's no bennefit if you are looking for improved MPG in town. The overdrive is used for highway only.

As other's have stated, it allows you to run a taller gear (such as the 3.42, 3.73, 4.11) for improved acceleration off lights or the starting line at a racetrack without sacrificing too much in MPG's on the highway.
 

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I have a 700R4 in my Nomad. It has a 327 and a 3:55 posi. When I get on the tollway or open road I put on my torque converter lock-up. (OD) At 65 MPH my RPM is under 2 grand, closer to 1700. If under 55 MPH I rarely use it. It goes down the highway great. I'm glad I have it.
 

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Overdrive will help you achieve the RPMs you want at cruising speed out on the highway. It will not help mileage in town when not traveling at sufficient RPMs to engage OD in high gear.

A 700R4 has an OD ratio of 0.70.

Richmond Gear has a pretty good calculator here (click on 'Street Performance')......

http://www.richmondgear.com/index.php/gear-transimission-calculators

Here in Texas, most highways speed limits out on the open road are either 70 MPH or 75 MPH. I will be having my 3rd member rebuilt in the next several months and installing an overdrive transmission, so I am deciding which rear gear ratio to go with.

I want to turn somewhere in the 2,000 - 2,200 RPM range when cruising at 70 MPH. My tires are 26.5" tall. I plan on installing the aftermarket T-56 transmission I have here with 0.62 6th gear. But if my bad knees force me to give up the clutch, I will reluctantly install a 700R4 automatic with a lockup converter.......

With 26.5" tall tires cruising at 70 MPH in high gear......

4.11 rear gears with .62 6th gear (T-56 transmission) = 2,262 RPMs
3.70 rear gears with .62 6th gear (T-56 transmission) = 2,036 RPMs
3.55 rear gears with .62 6th gear (T-56 transmission) = 1,953 RPMs

4.11 rear gears with .70 4th gear (700R4 transmission) = 2,553 RPMs
3.70 rear gears with .70 4th gear (700R4 transmission) = 2,299 RPMs
3.55 rear gears with .70 4th gear (700R4 transmission) = 2,206 RPMs

Happy Motoring,

Harry
 

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I just had a 700r4 installed because I was tired of my 4 speed, now I have 4.56 gears and wow what a difference. Now where I live I have a lot of highway driving so it was a big factor in my choice of trans. It used to run 3.70 gears but blew them up so that is how I ended up with the 4.56. If you are already running a low gear ratio you mat not see any difference, but the benefit of the OD is when you have a taller gear set.
 

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Discussion Starter · #13 ·
thanks every one of you for your helpful reply s,so i am continuing with the swap,and as you guys said i will get faster starts and better mpg on the highway[about half of my driving ]
 

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I built a Studebaker once, it ran a 350/350, and an 8 inch with 3:55 gears. When I was runnning 80 mph with the air on, it ran hot and loud. I swapped in 700R4, and the rpms came down, I think from about 3200 to under 2500. The car ran cooleer and quieter. That was all the proof I needed.
My 56 has a 350/700R4 and a 10 bolt with 3:42 gears. Should get the same kind of performance.
My 71 C30 pickup has a 402/700R4 with 4:10 or 4:11 gears. I've got a 3:54 geared Dana 60to put in it. Right now, it shifts out of first gear by about 5mph. I'm thinking the 3:54 will let it stay in first a little longer, making it a more useful gear, and reduce highway RPMs at the same time.
 
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