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Here is the formula to calculate rear end ratio:
  • Here is the formula to calculate rear end ratio:

  • =(60*RPM)/(Tire Size*Xmission Final Ratio*Road Speed)

  • Here's what you need to know:

    • Your Tire size in a number represented as "revolutions per mile"

    • All tires can be slightly different and you would have to look up your specific tire make and model to know the true number, but most of the time I use generic numbers and that will get you close. In the next step I'll show you the numbers I use for each tire size.

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Here are the numbers I use for the different tire sizes
  • Here are the numbers I use for the different tire sizes

    • 11R (Tall) 24.5 = 473 revolutions per mile

    • Low Pro 24.5 = 496 revolutions per mile

    • 11R (Tall) 22.5 = 496 revolutions per mile

    • Low Pro 22.5 = 512 revolutions per mile

    • MIchelin 445 Wide Single = 527 revolutions per mile

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Then you also need to know your transmission final ratio.
  • Then you also need to know your transmission final ratio.

  • Transmission Final Ratios that I use:

    • Direct drive = 1

    • Single OverDrive = .85

    • Double OverDrive = .73

  • ** These numbers may not be exact, but they are usually close enough for the calculation, MOST transmissions will be a double overdrive, and will use a .73 final ratio for the calculation.

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Now take your engine RPM at a given road speed and you'll have all 4 numbers you need for the calculation, lets use:
  • Now take your engine RPM at a given road speed and you'll have all 4 numbers you need for the calculation, lets use:

    • 1325 RPM at 60MPH

    • Low Pro 22.5 (512)

    • Double Overdrive 13 speed (.73)

  • ** Don't confuse the first number 60 with our road speed, the first number in the equation is ALWAYS 60

  • Do the math and you'll see that your rear end ratio is 3.55

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Be Safe, Be Profitable, and Master the Journey

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Finish Line

12 other people completed this guide.

Kevin

Member since: 11/02/2011

5,669 Reputation

4 Guides authored

One Comment

That was great. I am going to check the tach at 60mph later...but from what I can remember, it looks like our rear ends are 2.63 based on rpm's of 1350 on a direct transmission 10 speed and 22.5 lo-pro tires. But I will double check that 1350 number. This could be the reason our truck is strong but won't pull the hills. It's a "rollscanhardly" for sure. Thanks for the help.

Jenni and Rob Kebler - Reply

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