200r4 rear diff gears

Discussion in 'U-shift em' started by jaye, Nov 20, 2021.

  1. jaye

    jaye Well-Known Member

    I’m helping my friend out, he has 3.25 rear and want to go with a 200r4 transmission. I forgot what was the lowest recommended gears. He also have 20” wheels would the 200r4 be a good choice.
    Thanks Jaye
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The 2004R has a .67 OD 4th gear, and lock up converter. That would bring down the 3.25 to 2.18 Final Drive.

    20" wheels is not enough information. You need the actual tire size to figure out overall tire height.

    336/tire height X gear X MPH = RPM

    Assuming stock tire height of 26.6"

    336/26.6 X 2.18 X 60 MPH = 1652 RPM in lock up. Is this too low? Depends on the engine. If it is stock, that fine. If it has a big cam, maybe not so much.
  3. jaye

    jaye Well-Known Member

    They’re 275/35R20 rear, 255/35r20 front, thanks, no big cam.
    Last edited: Nov 20, 2021
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Rear tires is all that counts. P275/35R-20 are 27.6" tall

    336/27.6 X 2.18 X 60 MPH = 1592 RPM.
  5. sean Buick 76

    sean Buick 76 Buick Nut Staff Member

    Most of this comes down to the use of the car. I have two cars with 2004R trans, two different uses.

    My street and highway driver has a 2.56 gear, 27.5” tall tire and a 2004R. I like to drive it fast on the highway so it’s a good combo for fuel mileage and low rpm at speed. It turns just 2300 RPM at 100 MPH. That being said I don’t really care about off the line acceleration so it’s perfect for my use.

    my other car is setup for drag racing and it runs a 32” tall tire, 4.56 gear and 2004R trans
    . that one runs 3500 RPM at 100 MPH. That one tries to rip your head off, tons of fun, but I don’t highway drive it much so it’s fine for that.

    In general, a 4.10 or 3.73 gear is the best match for both acceleration and fuel mileage.
  6. philbquick

    philbquick Founders Club Member

    Turbo Trans Am's come with 200R4s and 3.23 rears, keep your tires under 28" and you'll be fine.
  7. knucklebusted

    knucklebusted Well-Known Member

    The thing to do is pick your best RPM you want to cruise at and make the rear gears and tire height work for that. Pick those variables and we can give you a much better idea where you gear it.

    The other good news is the lower first gear of the 200-4R will make those 3.25 gears launch like 3.59 gears and cruise like 2.18.

    I have an online spreadsheet you can use to play with the numbers. There's a tab for tires, rear gear/RPM and one for transmission comparisons.

    I this this should be public and you can access it but you might have to sign in to Microsoft: AutoMath.xlsx - Microsoft Excel Online (live.com)
    Dano likes this.
  8. Mart

    Mart Gold level member

    You go to the 2004r for better highway cruising with performance gearing. Run 410's period. That'll do everything you want.....
    Dano and Fox's Den like this.
  9. jaye

    jaye Well-Known Member

    Thanks for your input guys, he has a 64 Impala with a 4 speed and want to go to an automatic transmission due to knee issues. He doesn’t want to change the rear end gears or tires, just want to see if a 200r4 would be good the car is driven on the interstate
    Thanks again guys
  10. Fox's Den

    Fox's Den 27 years of racing the same 355 Buick motor

    I run a 373 gear 275-60-15 (28") tires I am at 2200 rpm at 65 locked up on highway 3000 stall convertor

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