TO FIND REAR AXLE RPM AND JACKSHAFT RPM

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TO FIND REAR AXLE RPM AND JACKSHAFT RPM AT A CERTAIN SPEED IN MPH
By Richard Schmidt

You will need to know what the rolling tire circumference is so you can determine what the axle rpm is.

You can easily find the rolling circumference by inflating your tire to the pressure that it will be when hot.
- Set tire pressure
- Put car on ground, level flat hard surface is preferred.
- Place a mark on the side of the tire close to the surface that contacts the road.
- Roll the car so that the mark is exactly at the lowest point on the tire.
- Make a mark on the ground opposite the mark on the tire.
- Roll the car forward until the mark is again closest to the ground.
- Make another mark on the ground.

Measure the distance between the marks on the ground, this is your rolling distance for one revolution of the tire.

Knowing what the axel rpm is at a given speed (mph) you can then calculate the jackshaft rpm at that speed.

1 mile = 63,360 inches
Example: rolling distance is 56.4 inches
y = constant

rolling distance = y
mile in inches

Example: 56.4/63360 = .0008901
y = .0008901


To find axle rpm:

mile in inches = revs per mile
rolling distance

Example 63360/56.4 = 1123.4 revs/mile

y * 60 = z

60 is number of minutes in an hour z = constant to be used to find MPH or RPM

Example: .0008901 * 60 = 0.053406

z = .053406

Axle rpm can now be found:

rpm = mph
~~~~~~ z


Example: lets say that we want to know what the axle rpm is at 75 mph, so:

75 / .053406 = 1404.3 rpm

Example: lets say the we know the axle rpm but want to know how fast we are going:

z * rpm = mph so .053406 * 1200 = 64.0872 mph


Now that we know what RPM the axle is turning at a given speed, we can now calculate the jack-shaft RPM based on the gear ratio.

If the gear ratio is 3.575 to 1 and we know that the axle is turning at 1404 RPM at 75 MPH, then it’s 1404 * 3.575 = 5008 RPM