How to Calculate a Ring & Pinion Gear Ratio

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The rear end of a car contains two gears called ring and pinion gears. The ring gear is round with teeth on one side. The pinion gear can be best thought of as a spear that has teeth that spiral up and around it. These two gears mesh together in the rear end to move the vehicle. There are times when you'll want to determine the gear ratio. This is most common in racing applications. The gear ratio can determine whether the car has quick acceleration or higher top-end speed. Determining a gear ratio is a fairly easy process that involves a little work and a little math.

Things You'll Need

  • Grease pencil
  • Jack up the car if the rear end is still installed. Place the car on jack stands to keep it from falling before getting under it.

  • Drain the gear lubricant from the rear end if it's still together. This is easily done by placing a pan under the rear end and loosening several bolts in the center of the rear end housing on what is referred to as the "pumpkin," the spherical center of the rear end, until the oil starts to drain.

  • Remove the ring and pinion gears from the rear end once the oil is drained.

  • Count the number of teeth on each gear and write these numbers down. Use a grease pencil to mark the ring gear where the count was started, so the teeth are not counted twice.

  • Divide the number of teeth on the ring gear by the number of teeth on the pinion gear. If you have 37 teeth on the ring gear and 9 on the pinion then divide 37 by 9. In this case you have a ratio of 37/9 = 4.11.

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