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Thread: gear ratio

  1. #1

    gear ratio

    i am going to change engines from a 355 to a 421,i have a choise of rear gears,456 or 486,car weighs 2780,power glide trans 14-32 tires.

  2. #2
    Senior Member RACING JUNKIE
    Join Date
    Jan 2006
    I'm assuming you're running 1/8 mi only. I think the 4.86 would be quicker, but if you're also running 1/4 mi, you might be twisting the 421 a little tight.



  3. #3
    Junior Member APPRENTICE
    Join Date
    Mar 2007
    I agree with zipper. the 4.56 would be a gear that you could run both 1/8 and 1/4 without issue. However, if your mostly 1/8 mile go with the 4.86.

  4. #4
    Senior Member SENIOR BUILDER
    Join Date
    Dec 2005

    RPM formula

    this is an easy formula that seems to work....

    MPH x Gear Ratio x 336 divided by Tire Diameter(inches)

    say 100 mph, 4.86 gears, 30" tires.....= 5443 RPM

    if you want to get fancy, you can add an inch or whatever for Tire growth, and add 5-10% for convertor slippage.....anyways, it works!

  5. Racing Junk

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