How to Figure Out the Gear Ratio of Your Car

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You typically hear people talking about gearing or gear ratios when speaking about muscle cars. Gear ratios can be changed in many vehicles, which can result in a change in the way that your vehicle drives. The gear ratio is determined by the final drive gears that are installed in your vehicle. If you plan on doing any upgrades on your car or truck, knowing how to determine what the gear ratio is can help you to make an educated decision when making changes to your drivetrain.

Part 1 of 5: Understanding gear ratios

set of gears pointing out the pinion

When talking about gear ratios, it is important to know that all vehicles have a gear ratio designed into the drivetrain of the vehicle. This ratio cannot be changed on most front wheel drive vehicles, but can be changed on many rear wheel drive vehicles. The gear ratio is the difference in the size of the pinion gear, or input gear, and the ring gear, or output gear. An example of this would be a gear ratio of 2:1. This would mean that the input gear would have two revolutions and the output would have one revolution. This results in a multiplication of torque, or the turning force at the wheels. This can affect the vehicle in many different ways:

  • A higher gear ratio can give your vehicle better acceleration.

  • A lower gear ratio can affect fuel mileage by decreasing engine RPM at highway speeds.

  • Gear ratios can be changed to compensate for changes in tire size.

Part 2 of 5: Preparation and lifting the vehicle

There are several ways to determine the gear ratio on your vehicle. It is important to have all of the correct materials and safety equipment prior to checking the vehicle.

Materials Needed

  • Chalk or paint pen
  • Differential cover gasket
  • Differential oil (manufacturer recommended)
  • Hydraulic floor jack
  • Jack stands
  • Oil catch pan
  • Ratchet w/sockets
  • Torque wrench
  • Wheel chocks

There are two different ways to determine your gear ratio. One of the methods will tell you the exact gear ratio and the other will give you an approximation. In both instances, you will prepare the vehicle the same way.

Step 1: Park the vehicle on a flat, hard, level surface.

Step 2: Place your wheel chocks around the front tires of the vehicle.

corvette jacked up on chocks

Step 2: Using the floor jack, lift the vehicle at the manufacturer's recommended lifting points. Raise the car until the back tires are off the ground.

Step 4: Place the jack stands under the jacking points. Then lower the vehicle onto them.

Step 5: Put the vehicle into neutral so that you can spin the driveshaft by hand.

Part 2 of 5: Determining an approximation of your car’s gear ratio

white line placed on the driveshaft near the differential

  • Tip: You need to position yourself so that you can grasp the driveshaft and see the inside of the rear tires at the same time.

backside of tire with white line on it

Step 1: Place a visible mark on the driveshaft near the differential housing. Use the chalk or paint pen. Also place a mark on the back of one of the rear tires.

  • Tip: You need to be able to watch both marks during this inspection.

hand spinning driveshaft

Step 2: Spin the driveshaft by hand. While spinning the driveshaft, count the revolutions by watching the markings, as well as counting the revolutions of the tire at the same time.

Step 2: Continue spinning the driveshaft. Keep counting the rotation of the marks you have made, until you feel like you have a good record of each rotation.

Step 4: Divide the number of revolutions of the driveshaft by the revolutions of the tire. This will give you your approximate gear ratio.

  • Example: If the driveshaft was rotated 12 times and the tire rotated four times, then that is a gear ratio of 2:1. If the driveshaft was rotated 12.5 times and the tire rotated four times, then that would be 2.125:1.

Part 4 of 5: Determining a more accurate gear ratio

This is the most accurate method of determining your gear ratio. Most gear ratios are not exact numbers, making the spinning method less accurate.

oil catch pan under differential

Step 1: Place the oil catch pan under the rear differential cover.

rear differential cover

Step 2: Remove the bolts that hold the rear differential cover on. Use the appropriate socket and ratchet.

Step 2: Remove the rear cover and allow the oil to drain.

numbers stamped on ring gears

Step 4: Find the numbers on the ring gear. Spin the ring gear by hand until you locate the numbers stamped onto the ring gear. These numbers indicate your gear ratio.

Step 5: Clean the cover mounting gasket on the cover and the housing.

Step 6: Install the differential cover using the new gasket.

Step 7: Tighten the differential cover bolts down using the torque wrench. You will need a service manual in order to know the correct tightening specifications.

Step 8: Fill the differential oil to the correct fluid level. See the manufacturer’s specifications for the correct level.

Part 5 of 5: Complete the process

Step 1: Place the vehicle in park and apply the parking brake.

jack stand pulled out from beneath the car

Step 2: Lift the rear of the vehicle with the floor jack and remove the jack stands.

Step 2: Lower the vehicle to the ground and remove the wheel chocks.

When making modifications or upgrades to your vehicle, knowing the gear ratio can help you to make a knowledgeable decision. Changes can be made to your ratio depending on the type of improvement that you are looking to make. Keep in mind that on some computer-controlled vehicles, the new gear ratio must be programmed into the vehicle’s module to ensure that the speedometer and shift points of the transmission are correct. If you are uncomfortable with any parts of this process, you can have a certified mechanic from YourMechanic come and remove the differential cover and get the gear ratio for you.


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YourMechanic Differential/Gear Oil Replacement Service

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Number of Reviews

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Chris

20 years of experience
1612 reviews
Chris
20 years of experience
Lexus GS300 - Differential / Gear Oil Replacement (Rear) - Irvine, California
Chris is an excellent mechanic. Always on time! A mechanic you can trust because of his work ethics and taking the time to diagnose your car.
Jeep Wrangler - Differential / Gear Oil - Mission Viejo, California
Chris did a great job we are very happy. He was very knowledgeable and very professional. He took care of all our mechanical issues and our car has never run better. He left the premisses spotless. We will definitely request him again.

Andrian

30 years of experience
73 reviews
Andrian
30 years of experience
Nissan 300ZX - Differential / Gear Oil Replacement (Rear) - Pompano Beach, Florida
On time, extremely professional, and highly knowledgeable. Great overall experience and will certainly count on him in the future for other services.

Ernesto

30 years of experience
27 reviews
Ernesto
30 years of experience
Honda CR-V - Differential / Gear Oil Replacement (Rear) - Hartland, Wisconsin
Ernesto called me to let me know he would be arriving late due to some repair issues with his first appointment. I received a text when he did arrive. I appreciated these courtesies! Ernesto was polite, friendly, knowledgeable, and efficient. The repair went well, and he explained the various procedures as he performed them. Previously, he worked as a Honda service mechanic, so I was extremely comfortable as he worked on my car, and he took some time to give me a heads-up about future maintenance issues I could likely expect. I would not hesitate to request Ernesto for future work, and I offer my highest recommendation for his work and professional conduct. Glad to have met you, Ernesto!

Antonios

16 years of experience
48 reviews
Antonios
16 years of experience
Toyota RAV4 - Differential / Gear Oil Replacement (Front, Rear) - Sharon, Massachusetts
I highly recommend Tony. It's the second service he has performed on my car. This time he made sure I had the right spark plugs that best fit my Toyota. If I would have replaced the spark plugs myself would get the highest rated ones which are not the OEM for Toyota.

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