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Thread: Speed Kills

  1. #1
    5 Star Lounger Maudibe's Avatar
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    Speed Kills

    The engine in car is spinning at 2350 RPMs as it travels down the highway in 4th gear. Its speed is 55 mi/hr. The differential has a 3.73 gear ratio meaning that the drive shaft spins 3.73 times for every 1 turn of the wheels.

    The enthusiastic owner wants to soup it up a bit and changes the rear to a 4.11 gear ratio or 4.11 turns of the driveshaft to one spin of the wheels. He also replaces the rear tires with slicks that are 2’’ more in diameter (outside edge to outside edge=28"). If the car travels in 4th gear at the same RPMs down the highway, how much faster/slower will the car be traveling?

    Good Luck,
    Maud

    Last edited by Maudibe; 2014-02-21 at 01:25. Reason: remove answer

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  3. #2
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    28" before or after teh switch ?

  4. #3
    5 Star Lounger Maudibe's Avatar
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    The slicks are 2" more in diameter (28") than before the change.

  5. #4
    5 Star Lounger Maudibe's Avatar
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    "Is there no one else?....Is there no one else?" Achilles (Brad Pitt) in Troy

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    Okay, let's try this.

    The increase in tire size will increase the circumference of the wheel by 1.0769231x, thus increasing the speed of the car to 59.23077 mph.
    The change in the gear ratio will reduce the wheel rotation to .907543x, reducing the speed from 59.23077 to 53.75445 mph, for an overall reduction in speed.

  7. #6
    5 Star Lounger Maudibe's Avatar
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    Very Impressive. I also figured it out using percentages but in a slightly more difficult method than you did. Nicely done!

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    Thank you! I was hoping I hadn't missed anything.

    Doug

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