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axle gear ratio???

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Balou, Apr 28, 2000.

  1. Balou

    Balou 1/2 ton status

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    Wondered how she could go 120 mph at 3200 rpm.
    SM 465, NP 205, 35x12.5
    Found 1.6 driveshaft revolution for 1 wheel revolution???
    This means 1.6:1 ratio, right ???
    Anybody heared of this ???
    Off-road is agreable (20.5:1 1st. low), highway is an adventure!!!

    Balou
    '79 K5,350,465,205
    [​IMG] get fuc.... M-Class out of my way !!! [​IMG]
     
  2. mudfanatic

    mudfanatic 1/2 ton status

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    OK if I remember right to gt your proper ratio you would jack up one side of your rig and rotate that tire around once, count the revolutions of the driveshaft then multiply that by 2 to get your approx ratio.
    so if in one rev of the tire your driveline turned 1.6 times then you multiplied that by 2 you'd get 3.2 the closest axle ratio would be 3.07 so that would probably be your ratio.
    am I wrong anyone? please tell me I am so that I don't have to go pull the diff covers to find my real ratio LOL
    '77 K5

    <font color=red>mudfanatic</font color=red>[​IMG]
     
  3. CBLAZER81

    CBLAZER81 1/2 ton status

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  4. walrus101

    walrus101 Registered Member

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    give me till tomorrow. i will talk to the guy that helped me figure mine. check billr response in post gear ratio?

    walrus101

    goo goo gaju
     
  5. 6.2Blazer

    6.2Blazer 1/2 ton status

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    Doing some calculations and assuming the tires are a true 35" diameter, 120 mph @ 3200 rpm would mean you had a 2.73 gear ratio. I believe this ratio was offered from the factory during some model years.
    I bet it feels a little sluggish off the line.......
     
  6. rich

    rich 1/2 ton status

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    I agree with MudFanatic - multiply the driveshaft rotations by 2 to get your gear ratio. I just did this on my Blazer the other week and it worked (I verified when I pulled the diff covers).

    The reason you have to multiply by two is that you're only turning one wheel and the other wheel is staying stationary, so the differentiail is causing the 2:1 change in ratio. If you have a friend help you, and the two of you are able to rotate both tires simultaneously (or if you have a functional posi or locker), you will get the true ratio.

    Richard
    '76 K-5 Blazer
    '90 K-5 Blazer
     
  7. jarheadk5

    jarheadk5 1/2 ton status

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    5000+ lb. truck...
    35" tires...
    350 small block...
    Aerodynamics of a billboard...
    120 mph...

    That better be one stallion of a 350!!

    I hate to think of what 35" offroad tires would do at 120 mph without bulletproof axle location and suspension. Can you say "tire shake"?? I'm thinking Baja 1000 truck at this point.

    Wow, this could be a lot of fun...

    OK, who wants to sponsor me and my new Blazer desert racing team?! LOL [​IMG]

    [​IMG] Semper Maintenance!
    [​IMG] So many ideas, so little money...
     
  8. blazer72

    blazer72 1/2 ton status

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    Been there done that but it does take a a real strong 350 and or a real good tail wind. The real thing I wan to know is when you put on the 35'd did you redo the speed drive so it reads right!
     
  9. CBLAZER81

    CBLAZER81 1/2 ton status

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    Ok,just figured my top speed out.118.??,with 35x15.5x15 Swamper SX's(not the most balanceable tire).I'm guessing at the rpm's but i figured it at 3500(was probably closer to 4000),going down hill(very long hill-20 miles?) with 3.08 gears.It actually smoothed out after about 85-90mph.I've since changed gears so it'll probably never happen again but thats probably a good thing.I will always miss that smooth Cadillac type ride though.

    [​IMG][​IMG]
     
  10. BB454

    BB454 1/2 ton status

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    Balou - That M-Class is hooked on your front bumper mate...lol!!!

    BB454
    IF IT DOES NOT HAVE 8 PISTONS - IT'S BROKEN !
     
  11. Andreas

    Andreas 1/2 ton status

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    Yup, if you have open diffs, you´ll need to turn the wheel 2 times when only one side is jacked up. Judging by your observations (1.6 revolutions at 1 turn) I think you have 3.07 gears. Not the best for 35"´s, but HEY, I ran 44" Gumbos with 3.07 (T-Case constantly in low ofcourse[​IMG])

    Andreas

    406cui of pure sh*t[​IMG]
     
  12. 6.2Blazer

    6.2Blazer 1/2 ton status

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    Well, if your 120 mph at 3200 rpm with 35" tires is accurate, you would have to have something in the 2.73 range for this. If you didn't correct the speedometer for the larger tires, than the vehicle would be going faster than the speedo reads so the change from stock tires to 35's would probably give you at least 5-10 mph variance (meaning you were going 125-130 mph). Also, tires listed as 35's are very rarely actually a full 35"tall, plus take into account the static rolling radius (due to the "bulge" in the tire with weight on it) makes it even shorter when calculating overall gear ratios...which means you would need a even higher (lower numerically) gear ratio to achieve that speed at the given rpm...........plus one heck of an engine........
     

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