Register now to get rid of these ads!

Technical Rear end ratio suggestion

Discussion in 'The Hokey Ass Message Board' started by txcr13, Jun 16, 2020.

  1. txcr13
    Joined: Feb 15, 2010
    Posts: 233


    Getting ready to replace the 2:73 gears that came in my 8.5" 10 bolt.
    Engine is 406 SBC, Muncie 4 speed transmission, 29" rear tires, 235 @ 50 roller cam, single four barrel.
    Car will never see the strip. Will only be driven occasionally.
    Thinking 3:42 or 3:55 gears, would like to hear what your experience.
    Yes, I will do the speed vs rpm calculations, but want to hear user comments.
  2. hemihotrod66
    Joined: May 5, 2019
    Posts: 177


    Think these would fine...
    Aeroman likes this.
  3. squirrel
    Joined: Sep 23, 2004
    Posts: 46,148


    or 3.73

    raise your hand if you remember when 3.73 was "highway gears"
  4. Register now to get rid of these ads!

  5. Illustrious Hector
    Joined: Jun 15, 2020
    Posts: 28

    Illustrious Hector
    from Alberta

    What kinda car? How are you driving it? highway, stoplight warrior? low speed cruise?
    31hotrodguy likes this.
  6. Mike VV
    Joined: Sep 28, 2010
    Posts: 1,934

    Mike VV
    from SoCal

    Agreed - 3.73 would be ideal.
    Making the engine struggle while leaving stop signs, or accelerating while going slow, while it's hard on the engine, it's no fun in the seat either.

    When 3.73 was a highway gear..?
    Hell, back in the early 70's, my girlfriend lived 30 miles away. My 56 Chevy had 4.11 gears in it for about a year and a half, then I finally found a GOOD highway gear...3.90's. I say "finally" found, at the time, no one was making the big Pontiac/Olds (I put a 57 Pontiac housing in the car) parts at that time. The 3.90 ratio was like having 10 pounds of platinum.

    Now a days, all of those parts are easy to find for the big GM axle.

  7. bchctybob
    Joined: Sep 18, 2011
    Posts: 2,224


    My '56 Chevy had 3.08s in it when I got it. It felt mighty lazy, even on the freeway at 65-70. At the first opportunity I bought some 3.55s and it woke it right up. Small block Chevys seem to like to stretch their legs. It pulls nicely from a dead stop and at highway speeds. If I run across some 3.70s for a reasonable price, I'd like to try them. The increased RPM takes a little getting used to when most of our late model drivers go quietly down the road at 1800 rpm, but with a full length exhaust system and quiet mufflers it's fine........and a lot more fun. I guess that's why 5 spds and OD automatics are so popular these days, you get the acceleration in the lower gears and the OD keeps the highway rpm down - the best of both worlds.
    Truckdoctor Andy and olscrounger like this.
  8. Mimilan
    Joined: Jun 13, 2019
    Posts: 570


    2800 rpm at 65 mph = 3.71:1 which is close ^^^ [245 deg @ .050 is pretty stout for cruising]
    2500 rpm at 65 mph = 3.31:1
    pitman likes this.
  9. For a while there, most general public 4speed cars came with 3.55 gears standard.
    loudbang, txcr13 and VANDENPLAS like this.
  10. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 931


    Or use a four speed box with a low first gear.
  11. rusty rocket
    Joined: Oct 30, 2011
    Posts: 3,830

    rusty rocket

    I have 3.00 in my 8”, t5, 350 in my 32 sedan. During The build stage I was sure I was going to change Gears but waited to get it running and driving. I’m glad I never changed it has enough poop to get it rolling and it will Cruze at 80 mph all day long.
    MMM1693 likes this.
  12. jimgoetz
    Joined: Sep 6, 2013
    Posts: 386


    I've got a 2.73 with a 350 turbo and 29in. tire in my T. Just like rusty rocket I planed on changing it but it works better than I thought it would. It's a pretty light car though. DSCN1196.JPG
    pitman and treb11 like this.
  13. My Mercury came Stock with 3.73 Standard Shift/ when I put the SBC in it
    with the Automatic, I didn't change the Rear it is Great on the Highway
    and Coming off the Line.

    Just my 3.5 cents

    Live Learn & Die a Fool
    loudbang likes this.
  14. I have a 2.52 1st gear M20 and run a 3.89 rear, works for me!
    loudbang likes this.
  15. pitman
    Joined: May 14, 2006
    Posts: 4,923


    M-20, low first is the ticket.
    4th, at 3.08 or 3.23, 'overdrive'.
    loudbang likes this.
  16. jimmy six
    Joined: Mar 21, 2006
    Posts: 6,413

    jimmy six

    I put 3.25’s in my sons 40 Chevrolet with a 302 GMC. I told him to get a 4 speed with a lower 1st gear. He bought a Richmond with a 2.88 and it’s perfect. A transmission can dictate the rear ratio.
    pitman, txcr13 and Hnstray like this.
  17. paul philliup
    Joined: Oct 3, 2013
    Posts: 182

    paul philliup
    from ohio

    Back in the late 60"a and early 70's I ran 4.89's with my 4speed 66 Sport Fury . IMG_20200121_183335546.jpg
    Elcohaulic and loudbang like this.
  18. indyjps
    Joined: Feb 21, 2007
    Posts: 3,834


    With that duration, Id say 3.55 is minimum, 3.73 or 4.11 is better. Itll suck on the highway but thats the tradeoff with a healthy cam.
    Be honest with yourself how you wanna use the car. If you want more of a driver, higway cruiser, swapping cams would be worth it.

    Or just spin it hard on the highway and throw rings and bearings in when needed. Its not like its a daily driver, use it up, beat on it, SBC parts are cheap.
    427 sleeper, bobss396 and loudbang like this.
  19. txcr13
    Joined: Feb 15, 2010
    Posts: 233


    1946 Studebaker coupe. Occasional drive/cruise, but wont be beating on it. Will open it up to hear it through the gears every now and again. Will not see many freeway miles.
  20. txcr13
    Joined: Feb 15, 2010
    Posts: 233


    Thanks, good advice.
  21. Black_Sheep
    Joined: May 22, 2010
    Posts: 1,177


    Generally speaking 3.55 is a good compromise for city / highway driving.
    scotty t, loudbang and txcr13 like this.
  22. hemihotrod66
    Joined: May 5, 2019
    Posts: 177


    HaHaHa they still are in my hotrod.....
    loudbang likes this.
  23. lippy
    Joined: Sep 27, 2006
    Posts: 4,609

    from Ks

    If you have an engine that makes power down low, under 5,000, then you need a higher gear like a 3.50. Or a 3.08. Lippy
    pitman and Elcohaulic like this.
  24. Lil32
    Joined: Apr 4, 2012
    Posts: 1,338


    350 Chev
    3.25 gear 9 inch. 350 turbo
    2200 rpm at 110Ks
    gives me a bit of performance and good fuel milage
    pitman, mgtstumpy and loudbang like this.
  25. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 931


    There's a lot to having a 2 series rear end with a top 1:1 gear and a torque cam. The stator will multiply torque to a higher degree with a higher (lower numeral) rear end. On the street, you need big torque right off idle not up in 5 or 6 thousand rpm..

    I ruined the performance of many good running motors putting long duration, short lobe separation, rump rump, cams in them and then going into a state of denial about how lousy they really ran..

    My present set up is a 1978 454 stock compression, stock cam, th400, 2;73.. This set up screams out of the hole and cruises at 90 all day long with ease... Tuning it made a world of difference, 20 initial, 16 mechanical all in by 2600, no vacuum.. You can add a lot of initial with low compression and still run 87. Im running a 180 t stat, I might go to a 160 if I get any ping.
    Hnstray and bobss396 like this.
  26. I had the stock 3.56 rear in the car and it lugged badly around town unless I left it in 3rd. I did some calculations and came up with the 3.89 ratio. I have tires that are close to 29" tall and at 70 I'm turning around 3200 RPMS.
    Elcohaulic likes this.
  27. missysdad1
    Joined: Dec 9, 2008
    Posts: 2,934


    The most important lesson I have learned about gears in hot rods that are regularly driven for pleasure is that the factory set-up is usually pretty close to ideal. Meaning a final drive ratio of something between 2.75:1 and 3.5:1 is going to give you the most satisfactory compromise between performance, mileage and comfort.

    I recently removed the factory 2.75:1 gears in the 8-inch under my tub (30.5" tires) and replaced them with 3.80:1 posi gears thinking that with a TH350 and no overdrive that this would be a really great combination behind a relatively mild SBC. After all, I reasoned, this has been the "go-to" gear ratio for generations of street-driven performance cars, hasn't it?

    Well, maybe so, but I was very disappointed in the driveability of the car with the 3.80:1 gears in place. The combination was oddly uncomfortable to drive, always at odds with itself at low speeds and very busy at freeway speeds. Not at all what I wanted in a car that gets driven almost every fair-weather day.

    So, I pulled the 3.80:1 gears out and replaced them with 3.25:1 gears - half way in between the former 2.75:1 stock gears and the 3.80:1 gears I had put in to replace them. This has proven to be ideal for my car and my driving conditions. The car is much happier and I am much happier now as well.
    pitman and Elcohaulic like this.
  28. I have to add my 3.5 cents.!
    If you Change your Rear Tire size you change your Rear Size.!

    Just my 3.5 cents

    Live Learn & Die a Fool
  29. squirrel
    Joined: Sep 23, 2004
    Posts: 46,148


    For a way to visualize how tire size affects "gearing", think of a car with 27.5" tall tires, and 2.75:1 rearend gears. If you went to 30.8" tall tires, you'd have to go to 3.08 rear gears to keep the same RPM at MPH. Going to 32.5" tires (you're getting into 4x4 territory here), you'd need to move up to 3.25 gears. Dropping to 25" tires, you'd need a 2.50 rear gear.

    You can see that drastic changes in tire size, equate to relatively mild changes in overall drive ratio. So, usually changing gears by 10% or so will get you noticeable results, while changing tire size a little won't really do anything.
    Country Joe and jimgoetz like this.
  30. GearheadsQCE
    Joined: Mar 23, 2011
    Posts: 2,475


    I've never seen that proportional comparison, tire size/gear ratio. That provides a quick reference that I will definitely remember.
    My mentor (Joy Fair) in oval track racing and the guy that got me started in QCs said that the average race car driver can't tell the difference in ratio changes of less than 1/4 ratio. He proved this by the following story.
    Joe Ruttman was driving for Fair while he healed from a broken back. Big Joe insisted on a .1 gear change. Fair got under the car and banged around for a while. Ruttman went out and improved his lap times. Came in and said, "See!"
    Fair never changed the gear :D
    Truckdoctor Andy likes this.

Share This Page

Register now to get rid of these ads!


Copyright © 1995-2020 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.