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Thread: Hey automatic tranny guys

  1. #1
    herb fraser's Avatar
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    Hey automatic tranny guys

    As you may know I am using a 350 sb chevy , putting out about 310 hp, and a T350 tranny with 3.73 gears in the rear end. Last nite I went for a go cart ride and it was way to easy to spin the tires , I am thinking that I should change out the rears for a set of 3.08s
    Most likely to many RPMs at high way speed . Therefore more noise and less gas millage .But mostly concerned about the jumping off from start, kinda want something better to drive, maybe I am wrong?

    Any thoughts would be helpful
    Thanks Herb

  2. #2
    Senior Member
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    Herb
    If you are going to do any road trips like power tour etc. you should use an online calculator and just fill in the blanks.
    Think about cruising at 60 70 and 80 as the most common scenarios and look at the different RPMs for those speeds.
    You just plug in your tire size, different rear gear ratios and final drive ratio for the TH350, there shouldn't be enough
    loss from the TQ convertor to change much unless you get something wild for stall speed. You want to choose the
    sweet spot so you aren't lugging the engine much below 1800 or buzzing it much above 3000 for those hwy speeds.

  3. #3
    PLATNUM Supporting Member
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    IMO 3:73 rear gear is a bit much for the street with a 350 trans. You will be turning a lot of RPM's. Lots of guys down here with 350 Chevrolet engines in rods use the 700R4 overdrive GM transmission. With your setup I bet that truck is super fast right off the line though.

  4. #4
    herb fraser's Avatar
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    Quote Originally Posted by wallace18 View Post
    IMO 3:73 rear gear is a bit much for the street with a 350 trans. You will be turning a lot of RPM's. Lots of guys down here with 350 Chevrolet engines in rods use the 700R4 overdrive GM transmission. With your setup I bet that truck is super fast right off the line though.
    Crazy fast off the line Hindsight is great , I would of use a tranny with overdrive, so I guess the cheapest way to go would be to change out the gears, not really worried about lots of speed off the line .

  5. #5
    Seasoned Citizen NAZ's Avatar
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    Unless you're planning to run some large diameter rear tires, even the 3.08:1 rear gear ratio may give more RPM at cruise than you'd like. Run the numbers before you decide.
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  6. #6
    herb fraser's Avatar
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    just ran the numbers and with my 32 inch tall tires with 3.73 gears 2300 rpm at 60 with 3.08 gears 1900 rpm , doesn't seem if 2300 rpm is to bad and don't know if it worth changing gears to get the rpm down to 1900

  7. #7
    herb fraser's Avatar
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    Quote Originally Posted by NAZ View Post
    Unless you're planning to run some large diameter rear tires, even the 3.08:1 rear gear ratio may give more RPM at cruise than you'd like. Run the numbers before you decide.
    Yes I am running tall tires they are 32 inch tall compared to my tires on my cobra which are 26 tall
    Last edited by herb fraser; 01-11-2019 at 06:09 PM.

  8. #8

    Steve >> aka: GoDadGo
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    Herb,

    32" tires with 3.08 gears will yield 2,265 RPM at 70 MPH with ZERO slip.
    32" tires with 3.73 gears will yield 2,745 RPM at 70 MPH with ZERO slip.
    You can pretty much add in 5-10% slip based on your converter choice.
    Hope this helps & Good Luck Fellow Chevy Fan!

    Steve
    Last edited by GoDadGo; 01-11-2019 at 08:31 AM.

  9. #9
    herb fraser's Avatar
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    Thanks Guys, at this point I am going to wait and see how I like it after it all done and on the road , Next build a tranny with an overdrive

  10. #10
    East Coast Speed Machines Erik W. Treves's Avatar
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    Fwiw... mine has 3.08s
    FFR 1879, Blown DSS 306,REDLINE management, VeryCoolParts Tuned 460RWHP

    FFR 818S, The Flash, Chassis #5, 2.0L, LSD, Electromotive TEC-S, VCP Tuned, 278RWHP 265 RWTQ

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    FFR 8335, Black Mamba, 289 FIA CSX 2001 tribute car, 347, 48 IDA webers, VCP Tuned, 311 RWHP 386 RWTQ, 3-link, Trigo's

    FFR 0004, Gen 3 , Hawk Coupe, Coyote twin turbo, 683 RWHP 559 RWTQ, IRS, VCP Tuned. "not too shabby"

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