How to Calculate RPM and MPH to Find Out whether You Need a Motor Reducer

motor reducerRPM is short for revolutions per minute and is a term used to describe how many turns a single object does in one minute. Basically, this is a measure of how quickly this object is turning around an axis. Many people think that RPM is a measure strictly used when it comes to vehicle engines but they are wrong. As a matter of fact, RPM is used to determine how well bullets penetrate and fly, how powerful a motor is, gear ratios, wind speed, etc. You might want to know the speed range within which your motor works best. If you have a motor and you want to calculate its RPM in order to find out whether you need to use a motor reducer or not, follow these steps:

  1. Measure the diameter of one of the tires on the driving end of your vehicle. You will have to take into consideration the weight of your vehicle so make sure to measure the tire from top to bottom.
  2. Take a look at your manual to find out what the axle gear ratio is and also what the transmission gear ratios is in the highest gear. Multiply one by the other in order to get the final drive ratio.
  3. Multiply the speed for which you would like to calculate the revolutions per minute by the value that you got for the final drive ratio. Then multiply that result by 336 which is a constant. After you have done this, simply divide your result by the diameter of your tire which you already measured during the first step.
  4. Now that you have the RPM, you can easily get the miles per hour. Multiply the diameter of the tire by the RPM. Use the same tire size and final drive ratio that you used in step 3 in order to calculate the speed. If you have 3,000 RPM, then 28 * 3,000 = 84,000. Multiply the constant 336 by your final drive ratio. For example, if your final drive ratio is 3.28 then, 336 * 3.28 = 1,102.08. The last thing to do here is to divide the first end result by the second one and you get 84,000 / 1,102.08 = 72.22 miles per hour.
Considering your vehicle has a working tachometer, you will not have to go through all of those calculations. However, it is useful to know these formulas when you have to work with vehicles without tachometers or ones that no longer work in order to find out whether a motor reducer is needed or not.

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