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Projects Best differential/gear ratio to run with a 327 sbc?

Discussion in 'The Hokey Ass Message Board' started by IzzyGarcia7, Oct 13, 2021.

  1. IzzyGarcia7
    Joined: Apr 3, 2021
    Posts: 14


    What's happening guys, looking for some wisdom and recommendations! Recently, the differential on my '62 Impala gave out. The mechanic hasn't opened it up yet, but it's not catching at all. It's paired up to a 235 with a powerglide, but eventually I want to beef it up to a 327 with a 3 speed. She's getting put away for the rain/winter coming up anyway, would now be a good time to swap the differential to something that'll handle the power of the small block later, or should I stick with fixing the stock diff? If it is a good idea to swap the diff now, which ones do you SBC guys use? What ratio is best if it's just going to be a street/highway cruiser, not a streetrod that needs too much low end power? Many thanks in advance! upload_2021-10-13_20-28-25.jpeg
    Deuces likes this.
  2. anthony myrick
    Joined: Sep 4, 2009
    Posts: 10,068

    anthony myrick

    That rear came with SBC impalas.
    It’s plenty stout for your set up in my worthless opinion
    3:08 gears might not be fun from red light to red light but make up for it on the interstate.
    Figure out what ratio you have currently first.
    jnaki, pitman, IzzyGarcia7 and 4 others like this.
  3. Moriarity
    Joined: Apr 11, 2001
    Posts: 23,082

    Staff Member

    yup, the same rear end they used in 409 Impalas.... there are ratios from 3.08 thru 4.88 available. You are going to run a stick shift with the 327? try a3.36 or 3.55 ratio,, if you are going to run an automatic then 3.08 as Anthony said
  4. redo32
    Joined: Jul 16, 2008
    Posts: 1,949


    What were you doing that you broke that with a 235?
    winduptoy likes this.

  5. tubman
    Joined: May 16, 2007
    Posts: 6,049


    It may not be totally applicable, but I have had two 327/350 Corvettes (C2's) with 4 speeds, one with a 3.55 and one with a 3.70. I liked them both, but if I had to pick one, it would probably go with the 3.70, because I could always go with a larger tire if it was to low (high numerically). Back in the day, I had a friend who bought a new 1963 Delray with a 340 hp 409 (4 BBL, juice lifter cam) with a 4 speed and a 3.08. Three years later (he had a rich uncle) he bought '66 Corvette with a 327/350, a 4 speed, and (again) 3.08 gears. Both of those cars were absolute pigs; I was with him one night when his Corvette got beat by a '62 Impala with a 327/250 and a Powerglide. (I think he had a thing about gas mileage.)

    4.11's are marginal for general use, and 4.56's totally impractical. That leaves (I think) the 3.23's which I have no experience with, but are closer to the 3.08's than the 3.55's, soooo...
    Last edited: Oct 13, 2021
    winduptoy and Deuces like this.
  6. Mimilan
    Joined: Jun 13, 2019
    Posts: 1,108


    That car of yours should have a 3.36:1 rear end ratio.
    And the powerglide in front of it has a 1.82:1 low gear and 1.0:1 top gear.

    With a TH350 you have a much lower 2.52:1 first gear so you could safely use a 3.08 ratio for highway cruising.

    A better choice for an "all rounder" is to use a 200-4R trans with a slightly lower 1st gear [2.74:1] and keep the existing 3.36:1 rear end ratio. This will help get it off the line.
    The 200-4R has a lock up converter and a 0.67:1 overdrive 4th gear.

    At 70 mph with 26" diameter tyres it would cruise at 2036 rpm in 4th gear.
  7. If all you need to get back on the road is rear axle repair, then that is all I would do.

    My '57 has a 3.50 behind a 327. I had 3.70 also which I liked until I hit the highway. It's not the fuel mileage, it's the engine wear that bothers for me.

    I have know idea of your skills or budget, but a 327/3 speed will grow to be more work than you might think.
    Shifter, clutch pedal, V-8 specific sheet metal. Some of this could be hard to find.
    IzzyGarcia7, big duece and Deuces like this.
  8. flatmotor40
    Joined: Apr 14, 2010
    Posts: 581

    from georgia

    My 55 has a 3.70 but I ordered a Gear Vendors OD for it this week .As my 40 has a 3.78 and I put a Mitchell OD in it and love it
  9. 55blacktie
    Joined: Aug 21, 2020
    Posts: 693


    If you plan to do much highway driving, I would not consider any ratio numerically higher than the 3.08 already mentioned. You might even consider a numerically lower ratio, depending on what's available. It doesn't sound like you care about how quickly you can get from stop light to stop light.

    My 55 Tbird/w Fordomatic transmission came with OEM 3.31 gears. Being used to modern cars/w overdrive, I didn't like the way it's geared when driving at highway speeds. Being that it has a Dana 44 rear axle, I had 2.72 gears from an 80s Corvette installed. Replacing the Fordomatic with a C4 that has Dynamic Racing Transmission's wide-ratio gear set/w 2.90 1st, will get it moving much better than the Fordomatic starting out in 2nd gear, and very close if manually shifting to low/1st (equivalent to having 3.29 axle/w Fordomatic).
    IzzyGarcia7 likes this.
  10. DDDenny
    Joined: Feb 6, 2015
    Posts: 16,059

    from oregon

    A 327/4 speed car loves a 3.55-3.70 rear gear.
  11. indyjps
    Joined: Feb 21, 2007
    Posts: 4,848


    3.36 and Th350 is a pretty straightforward set up and great daily driver. Powerglide to th350 should go in without driveshaft mods.
    Agree with 57 heap, look at all parts needed for manual swap before committing.

    200r4 and 3.73 gear is a very nice combo.

    If youre doing highway gears without a posi, you may get a takeout rear pretty cheap, look for guys doing full rear-end swaps. ( racers, pr*-t*uring)
    Truckdoctor Andy likes this.
  12. IzzyGarcia7
    Joined: Apr 3, 2021
    Posts: 14


    Wish I could tell you. Cruising through the city (Chicago) to meet up at a friend's place and she was fine the entire way there. I pulled up to a spot to parallel park, put it in reverse and she just wouldn't catch, bunch of grinding.
  13. 51 mercules
    Joined: Nov 29, 2008
    Posts: 3,431

    51 mercules

    I had 373's and a power glide with my 327. I liked it.
    Desoto291Hemi likes this.
  14. IzzyGarcia7
    Joined: Apr 3, 2021
    Posts: 14


    The swap is only going to come when the stovebolt gives out, which could be a long while from now considering how easy they are to quick fix, and it'll stay automatic. I'm just seeing if anything that breaks in the meantime could be swapped to something that'll be better compatible later on
  15. Mr48chev
    Joined: Dec 28, 2007
    Posts: 30,866


    I have to agree with what the others said, There is no reason to spend the money (BIG) to swap to a bigger rear end in that car with a 327 planned for it. 3.31 will give you the best of both worlds with fairly decent acceleration and not put a big dent in your wallet at the gas pump. With that 237 3.7 is going to cost you about 5 mpg from my past experiences with them. They didn't get better gas mileage over a 283 from the get go, they just were a hell of a lot cheaper to build and install to keep the price down.

    Remember that that rear end has a drop out center section. If a guy has another third member ready to go and the gasket you can honestly jack the car up, pull the wheels, pull the axles out far enough to clear, take the drive shaft loose and push it out of the way and drop the pumpkin and stab the other and be on the road again in under and hour. I knew guys in the 60's who drove to the drags with their 3.08 rear, swapped to 4.11 raced, swapped back and drove back quite often in the days when your daily driver was also your race car. Meaning that if and when you put a V8 in it swapping third members isn't a big deal if you don't like the gears you have.,
    jnaki, Mimilan and IzzyGarcia7 like this.
  16. IzzyGarcia7
    Joined: Apr 3, 2021
    Posts: 14


    It seems like the most recommended here is the 3.08 or a similar gear. How would this affect my current drivetrain if it is being swapped from a 3.36 that was mentioned above?
  17. mrspeedyt
    Joined: Sep 26, 2009
    Posts: 802


    if it turns out that the differential is ok you probably should just keep it. if the 3:36 is what you have… it is a decent compromise for normal driving. the 3:08 is fine too.
    pitman likes this.
  18. typo41
    Joined: Jul 8, 2011
    Posts: 2,572

    Member Emeritus

    End of the line, any rear end that fits will be good, from 8 inch Ford mustang to 8.8 Chevy.. take a tape measure to the local junk yard or use modern tech and make a number of calls
    Good Luck!
  19. Saturn V
    Joined: Oct 10, 2021
    Posts: 165

    Saturn V
    from Killen AL

    From my experience it all depends on the use of the car. If it's a show car or just a daily driver. I would use a 2.56 to 3.31 gear. If it see some strip time start dropping the ratio.
    I run Olds engines in most of my toys. So I usually put a 3.08 or higher gear in them for street driving and some mild bracket racing
  20. iagsxr
    Joined: Aug 26, 2008
    Posts: 171


    A. Fix the stock dif. You're not going to break it with a 327/auto.

    B. You understand a Powerglide and a TH350/400 are all 1:1 in high gear right? Whatever you turn for rpm now is what you'll turn after the engine swap +/- a small amount depending on any change in converter slippage. I.E. if you're happy with how it goes down the road now put the same gear back in it.

    C. I'm going to be the guy that says that thing'll be a dog with 3.08s in it. 3.36-3.55 would be my recommendation, but you do the math. There are online calculators that you can enter tire diameter, gear ratio, and speed to get rpm.
  21. Roothawg
    Joined: Mar 14, 2001
    Posts: 22,512


    I always pair my 327's with 3.36. GM did a lot of R&D to figure that out. With that said, my next 327 build will have a 3.90 gear paired with a 6 speed and a .64 OD.
  22. 55blacktie
    Joined: Aug 21, 2020
    Posts: 693


    iagsxr, you might want to include your definition of "dog." Although I'm sure it was lighter than a 62 Chevy, my 1988 Mustang 5.0/w AOD and 2.73 gears was no dog and capable of 25+ mpg driving on I5 from Bakersfield to Stockton. You might want to reread the OP. The owner isn't building a race car.
  23. iagsxr
    Joined: Aug 26, 2008
    Posts: 171


    Nope. Lighter car, fuel-injected 302, 2.40:1 first gear, not even comparable IMO.

    Like I said the calculators aren't hard to find, the OP should do his own math.
  24. chevyfordman
    Joined: Oct 4, 2008
    Posts: 1,151


    I like the 3:36's and 3:25's for ratios but then I'm a cruising type of guy with a 700r4.
  25. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 2,461


    Put a 3:08 or even better a 2:73, with a low compression 454 and a Th-400.. You will love it! Keep the 454 stock, keep the points and stock exhaust manifolds. And enjoy running 87 octane fuel while having big torque and the ability to put down most cars on the road..
    Run straight dual exhaust with Dynomax Super Turbo mufflers. No X pipes!
    Truckdoctor Andy likes this.
  26. I love that low end torque and smooth idle! The older I get, the more I find that stock is where it’s at, stock fixes a lot of little B/S problems.
    Elcohaulic and Mimilan like this.
  27. jnaki
    Joined: Jan 1, 2015
    Posts: 6,860



    This is always a topic of discussion. For the most part, the GM factory had the 3:55 gear with most older stock Chevy sedans during our time on the street and teenage years. Our teenage group had 3:55 to 4:56 and we all had different experiences with all of the ratios.
    But, from those experiences during our teenage years, when we bought the 327 powered 1940 Ford Sedan Delivery, it came with a Powerglide and 3:55 gears. It looked great and we bought it after a short drive. So, on the way home, it was handling like no other car that I had driven since 1958.

    It drifted, had a shimmy at higher freeway speeds and the brakes were just not grabbing when needed. It stopped, but one had to be prepared for a longer stopping distance. It looked nice, but it took months of rebuilding the whole front end and then had it professionally balanced, trued and aligned.
    The 327 power was good, but the newly purchased sedan delivery needed a lot of work to get it to a safe driving, road worthy hot rod. It was well worth the time, patience and work to get it running correctly for my wife and me for our So Cal hot rod experiences.

    We had no intention of racing the 327 sedan delivery. It was a cruiser, long distance road trip hot rod and a nice camper for two people. So, with the experience with a stock 3:55 gears from a long time ago, it felt right in all aspects of driving our hot rod sedan delivery. It was mentioned a 3:08 set of gears. That may be ok, but the lack of power for the whole set up on your Chevy sedan may not be what you would expect from a 327 powered motor.


    The 327 gave us the power we needed for the long mountain road climbs, the steep coastal surf trips and overall, a good handling sedan delivery at freeway speeds when we went to Long Beach twice a month to visit our family. The 3:55 gears made the 1940 Ford Sedan Delivery feel just right on those long 100 mile trips. With the power of the 327, it had enough go when the accelerator was pushed down. It was not the huge power surge of our old modified 1958 Impala, but, it was a cruiser, not a drag racing car.

    If we ever wanted a more powerful gripping surge when we accelerated, of course a 4:11 or 4:56 Positraction gear ratio was the best for the 327. But at the time, long distance driving with those two sets of gears would not be the best for the motor or two occupants. Our road trips were quiet, cool and comfortable without any worry about straining the motor using a 4:11 or 4:56 gear ratio. The 3:55 was just perfect for us. YRMV

  28. wheeldog57
    Joined: Dec 6, 2013
    Posts: 2,313


    @IzzyGarcia7 the 3.36 ratio was widely popular back in the day. Many still around and fairly straightforward to swap out.
    In my opinion you would barely notice a difference between a 3.08 and 3.36.
    Let know if you need a differential, I have a couple.
  29. oldsman41
    Joined: Jun 25, 2010
    Posts: 1,556


    3.08 should be ok with no overdrive depending on tire size
  30. My 58 Impala had a 3:55 rear it was good on trips as while as in town driving .

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