

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.



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



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.



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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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 lopro 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.