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Discussion Starter · #1 ·
Getting ready to put a Ford 9 inch in my 57, already have a ZZ4 and added a 700R4 a few years ago. I am running 235/70 15s on the rear. Anyone give me some insight on what gears to use on this rear diff addition?
 

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Depends on how you drive and what is important to you as far as the emphasis on performance/economy.
 

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Discussion Starter · #3 ·
Depends on how you drive and what is important to you as far as the emphasis on performance/economy.
I am looking for highway and in town operation vs racing it. Performance now with the stock rear end is OK for me, although with overdrive in it now, it could stand to gain a little better street performance.
 

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3.50 or 3.70 would be what most would prefer. 3.89 or 4.11 if you have tall tires, a big cam, or just more performance. 3.00 or 3.25 if your goal is gas mileage.
 

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I have the same set-up as you now and have a 3.36 in mine...Haven't driven it yet...However, I expect to have to move up to a 3.55, after I do start driving it..
 

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There isn't much difference between 3.36 & 3.55 in RPMs or takeoff to justify the cost in my opinion....3.73s is what most people use with that transmission for decent takeoff and cruise RPMs....
 

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Discussion Starter · #8 ·
Thank you all for your input. 3.73 is what I had in it tied to a 4 speed back in 72 but don't know that it still is since it was out of my hands for years enough to lose the 4 speed and gain a TH350.
 

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I am looking for highway and in town operation.
Depending on how much highway driving you are planning, what speeds you will be traveling at, and what rpm you want your engine turning, can be seen with a transmission ratio calculator.
Your 235/70 15 tires have a diameter of 28" and your 700R4 has a final gear ratio of .7
 

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There isn't much difference between 3.36 & 3.55 in RPMs or takeoff to justify the cost in my opinion....3.73s is what most people use with that transmission for decent takeoff and cruise RPMs....
With a low, low first gear of 3.06 on 700R's I shouldn't think takeoffs would be a problem even with 3.36 rear gears.

In fifty seven eight-cylinder engines used a 1.76:1 first gear ratio with powerglide transmissions and I believe a turboglide had a 2.67 first gear.
 

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1957 Chevy Bel Air Convertible; ZZ4 w HEI ign; 700r4; 9" rear w/3:50 gears; pwr disc fr w drum rear
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Same set up as yours. Happy with my 3.50 rear gear
70mph/ 2000 rpms
 
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