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Tire Size, Gearing . What Will MY RPMs be at.

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Monster, Aug 15, 2002.

  1. Monster

    Monster Registered Member

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    Is there a site where you can type in your gearing info and
    tire info to find out your Rpms of your motor

    If not my buddy has a TH350 and wants 4.56 gears to turn his 35 in tires. I also want 4.56 gears but I have a 700R4 and 35 in. tires. What Rpm will are motors be running at 65mph Thanks /forums/images/icons/confused.gif
     
  2. Blazer79

    Blazer79 1/2 ton status

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  3. mudhog

    mudhog THEGAME Staff Member Super Moderator

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    you would be ok with the 700r4 and 4:56's but he would be turning some high rpms at 65 /forums/images/icons/frown.gif
     
  4. 73k5blazer

    73k5blazer Unplug the matrix cable from the back of your head Premium Member

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    Remember too that most 35" tires are not 35" tall. Some are only 34" while most are probably 34.5, 34.8, etc...then you have to account for your rolling diameter, which is taking a height measurement (usally to the center point, a radius, then double it) with the weight of the vehicle on the tire. Now you down to high 33's or at best low 34's...all this can add up to another couple hundred RPM's at 65....just be aware....
     
  5. m j

    m j 1/2 ton status

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    I dont buy the loaded radius idea.
    the circumference is what you are rolling on, just measure that if you can and forget some magazines hairbrained math
     
  6. 73k5blazer

    73k5blazer Unplug the matrix cable from the back of your head Premium Member

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    No offense but that hairbrained math, as you call it, is what all OEM's around here use to calculate gearing, tires, engine performance, etc on their new cars and trucks. Not that I'm an expert, I'm in IT and have worked at Ford, GM and DCX, but my brother is a tire release engineer for GM, and he is the one who schooled me when I was deciding the gearing, tranmission, engine and tires for my truck. It's called a rolling radius. I never thought of using the circumference, I don't know what that would give. Measuring the diameter as the truck sits with weight is close, but still a wee bit high, measuring the radius from ground to center as the truck sits, then doubling it for diameter calculations is the real measurement.
     
  7. wrathORC

    wrathORC 1/2 ton status

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    http://www.smokemup.com/auto_math/mph_range.php

    You'll probably want to know the real height of your tires. I'd measure from the asphalt to th center of your hub and then double it.

    If you want to know what it really is at say 80mph then I'd add an inch to the height of the tire if you're running MTs. Tires grow, especially wide tires with lots of sidewall.
     
  8. dyeager535

    dyeager535 1 ton status Premium Member

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    Wire up a tach for temporary testing.

    Go off the advertised tire diameter to calculate, then after you are done, compute actual vehicle speed vs. the speedo reading.

    IMO, and my opinon only, measuring true tire diameter *accurately* is difficult enough (within 1/2") that its not worth trying to do so, and expecting it to be correct. Besides, +/- 200RPM is not going to make enough difference in engine speed to hurt or change anything, (economy, etc) it will only be a problem getting the speedo correct, which should be time tested or GPS'd anyways.
     

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