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  1. #1
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    Jul 2005
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    Default Calculating Revs Per Mile

    Radius - Diameter - Circumference

    To compute the diameter of a P-Metric tire, multiply the section width by the aspect ratio (as a percentage). If necessary, divide this number by 25.4 to convert millimeters to inches. For example, a 215/65 equates to a tire height of 5.5019685 inches (215 x .65) divided by 25.4. Multiplying this by 2 for the top and bottom sidewall height and adding the rim size will get the overall tire diameter. For example, if the tire is on a 15 inch rim, it is approximately 26 inches ((5.5 x 2) + 15). The radius is half of the diameter. Multiplying the diameter by pi (3.1415927...) will get the circumference.

    Tire Revolutions Per Mile



    To compute the number of tire revolutions per mile, divide 20,168 by the diameter.

    20168
    ---------------------------- = Tire Revs/Mile
    (Tire Heigth X 2) + Rim Size


    For a 215/65R15:

    20168
    ---------------- = 775
    (5.5" X 2) + 15


    For a 255/60R15:

    20168
    ---------------- = 747
    (6" X 2) + 15



    Speed

    By dividing the original tire revolutions by the new tire revolutions and multiplying by the indicated speed on the speedometer, we arrive at the new actual speed.


    (Orig. Tire Revs/Mile)
    ---------------------- X Indicated Speed = Actual Speed
    (New Tire Revs/Mile)

    Using the above tire sizes:

    (775)
    ----- X 60 mph = 62 mph
    (747)


    Rear Axle Ratio

    Changing tire diameters will also change the effective rear axle gear ratio. Divide the new tire revolutions by the original tire revolutions and multiply by the original axle ratio. For example, a 215/65s and a rear axle ratio of 3.42 (what else?) and moved to a 255/60 will net an effective ratio of 3.30.


    (New Tire Revs/Mile)
    --------------------- X Orig. Axle Ratio = Effect. Axle Ratio
    (Orig.Tire Revs/Mile)

    (747)
    ----- X 3.42 ratio = 3.30 ratio
    (775)

    As you can see, this will theoretically raise the top speed of the vehicle but
    hurt its acceleration.

    To regain the original effective ratio with the new tires, a 3.55 ratio is
    required.

    (Orig.Tire Revs/Mile)
    --------------------- X Orig. Axle Ratio = Equiv. Axle Ratio
    (New Tire Revs/Mile)



    (775)
    ----- X 3.42 ratio = 3.55 ratio
    (747)


    Maximum Speed

    To calculate the maximum speed of a vehicle, take:

    Overall Tire Diameter X Achieved Engine RPM (redline)
    ------------------------------------------------------ = Max Speed
    Differential Ratio X Top Gear Ratio X 336

    26" X 5250
    ----------------- = 177 mph
    3.42 X 0.67 X 336
    Last edited by Ted; 04-13-2008 at 02:02 PM.

  2. #2
    Join Date
    Jul 2005
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    Default

    Last edited by Ted; 07-19-2010 at 11:48 AM.

  3. #3
    Join Date
    Jul 2005
    Posts
    10,774

    Default Another Method...Revs per mile 9-3-09

    Here's the simple way if you don't already know how..

    Put a mark or piece of tape on the 6 o'clock position of your tire(side of it) and then make a mark on the ground next to it(starting point) , roll the truck forward or back one full turn so the mark is back at the 6 o'clock position again. make another mark on the ground. Now measure how far apart the 2 are in inches.. mine was 99"

    There are 63360" in a mile..

    so 63360/99 is 640 thats how many revs per mile the tire will make..

    Thanks to:

    Bcarriveau
    Moderator and Mafia #90 Diesel Garage..

    Great method....


 

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