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Determining axle ratiio

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  • Determining axle ratiio

    Ok, could not definitively find what I was looking for. So, if I'm beating a dead horse, I apologize. I have a 89 RS, 355, 700r4, 2000 stall, and the motor at flywheel pulled 410 hp at 5500. My question is I have two axles for it. One Eaton 373 drum axle and a 1999 z28 axle 342with disc. Which is better for the street and an occasional pass on a road course track. How do you calculate it and why would you use it? Thanks so much. Also need an aluminum front bumper bar for 3rd gen if you know anyone or anyone that can fab one.

  • #2
    It really depends on what you want, but you're not looking at a huge difference between the two.

    With a stock size tire, I imagine it'd be pretty "lively" with 3.73 gears. I'd guess you'd see 2400RPM neighborhood at 70 MPH. That would be 2200RPM at 70 MPH with 3.42 gears. If you have tires that will hook, 3.73:1 will yield better acceleration. Some depends on your engine. My stock LS1 is happy to cruise at 1700RPM at 70MPH in 6th gear, but 70MPH is more fun in 5th. 5th is .74 for me vs. .7 in your 700-4R, so I'm turning 2500 RPM at 70MPH. It's loud, but the engine is fine at that RPM.

    I'd probably lean toward 3.42 if you're going to drive it a lot, and especially if you're going to be on the highway. That 200 RPM difference at 70 isn't a lot anyway, but it might be noticeable on the highway, with loud exhaust. I will say that I am very glad I have 6th gear, which allows me to run just under 2000 RPM at 80MPH; it's SO much quieter. If you want to race, that's a different story. If you want to run a 28" tall tire, the final gearing with 3.73 would become the same as running 25.7" tires with 3.42.

    Just a couple notes.... Your '89 came with 4 wheels discs, didn't it? If it has discs in the rear, it'd be preferable to keep discs in the rear. You'd need a combination valve in the brake system to accommodate drums in the rear.

    vortek and MP81 like this.
    2000 Pontiac Trans Am WS6 M6 | C4 ZR1 Wheels | Borla Cat-back
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    • #3
      Thank you for the info. It kind of sounds like the 3.42 is going to better for me. 2200 is a lot nicer on the freeway vs 2400. Great info.

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      • #4
        It did have drum rear from the factory. I have a proportion valve from my old TA that had discs. I can go either way there. I think that the 3.42 would be better just because of that wouldn't you?

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        • #5
          Yes - and the 4th gen rears are a touch wider, which looks good on the 3rd gen cars.
          vortek likes this.
          Gone but not forgotten: 1986 Chevrolet Camaro IROC-Z

          "You shall ride eternal. Shiny and chrome."

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          • #6
            They are wider...you're right about that.

            3.42 is a great all around choice.
            DynoDave
            POCI # 72200



            1988 Pontiac Trans Am WS6

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            • #7
              Thanks again!

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