How to Calculate Gear Ratio | Autance

Ordinary motorists do not care so much about the gearing ratio of their cars. But if you’re in the business…

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How to Calculate Gear Ratio | Autance © How to Calculate Gear Ratio | Autance

Ordinary motorists do not care so much about the gearing ratio of their cars. But if you’re in the business of tuning cars or you’re a fan of muscle cars, then gear ratios are very important. This is a concept that can help you improve the driving performance of your vehicle. Learning what gear ratio is and how to calculate for it can help you build a high-performance car.

A Word About Gear Ratios

Cars use gear ratios in both the drive axle and the transmission for them to amplify the power from the engine. One cannot change the gear ratio of vehicles with front wheel drive systems; only on rear wheel drives.

There are two main components in any powertrain. These include the driver gear and the driven gear. The driver gear is that component that receives rotational force. In automobiles, the rotational force comes from the engine. The driver gear causes the rotation of the other – driven – gear. Some mechanics refer to them as the input gear and the output gear, respectively.

So, what is gear ratio? This is the relationship in the rates of rotational speed between the input (driver) gear and the output (driven) gear. For example, a simple 2-gear gear train that has a ratio of 2:1 means that the driver gear will have to rotate two times for the driven gear to make one complete rotation.

When applied to automobiles, the gear ratio is the relationship between the pinion gear and the ring gear. This is the equivalent of the input and output gears, respectively.

Calculating Gear Ratios

Calculating the ratio of any given gear train is easy enough. All gears have teeth. They interlock with the teeth of the adjacent gear, allowing it to turn in the direction opposite to the rotation of the driver gear.

In computing the gearing ratio, you only need to determine the number of teeth of both gears. For example, let us say you have a first or input gear that has 20 teeth and a second or output gear that has 30 teeth. Divide the number of teeth of the output gear by the number of teeth of the input gear. In our example, this is 30/20. This gives us a gear ratio of 1.5. What this means is that the first gear will have to turn 1.5 times for it to be able to turn the driven gear once.

The gears in our cars are not that simple, unfortunately. So the next question that people will often pose is how to find gear ratio in more complex gear trains. To do this, you can still follow the same formula when computing for the ratio of more complex gear trains. For example, a 3-gear system means you have an idler gear between the driver gear and the driven gear. Idler gears are often included to allow the driver and driven wheels to spin in the same direction. Idler wheels or gears can also change the plane of gear rotation.

Let us say the first gear has 7 teeth, the idler gear has 20 teeth, and the driven gear has 30 teeth. There are two ways you can compute this gear train’s ratio. First, divide the number of teeth of the driven gear (n=30) by the number of teeth of the driver gear (n=7). This will result in a ratio of 4.285 or 4.3:1. This means that the input gear will have to rotate 4.3 times to allow the output gear to make one complete rotation.

The second method involves computing for the gear ratios of the intermediate gears. This means you need to find the gear ratio between the input gear and the idler gear as well as between the idler gear and the output gear. In our example, this is 20/7 = 2.9 and 30/20 = 1.5. Multiplying 2.9 by 1.5 will yield 4.3.

The outcome is still the same as in the first method, which is 4.3. What is good about the second method is that you also get to have an idea of the ratio between the individual gears. This will also give you an idea as to why your transmission has different gear ratios as you move through the gears.

For example, a General Motors TH-200-4R automatic transmission can have a gear ratio of 2.74 in the first gear, 1.57 in the second, 1.00 in the third, and 0.67 in overdrive. If you notice, the gear ratio in the first gear is higher than the rest. This allows for better acceleration, drawing power from the engine. As the vehicle reaches its cruising speed, there is no need for very fast revolutions, so the gear ratio is lower. This can help improve fuel economy by reducing engine speeds.

How to Find the Gear Ratio of Your Vehicle

Learning to calculate the gear ratio is easy. Looking for it in your car can be a challenge, especially if you do not have your owner’s manual. Manufacturers always put the gear ratio information in their vehicle owner’s manual for reference purposes.

If you do not have the owner’s manual, then there are two ways you can find this information on your vehicle. In either method, you will need to raise your car high enough so you can work underneath it with greater freedom.

Make sure to park your car on a level, hard, and flat surface. Get your wheel chocks and secure the car’s front tires. Place an appropriate floor jack directly under the lifting points specified by the car manufacturer. Lift the car until the rear tires are a few inches off the floor. Get appropriate jack stands and place these under the jacking points on the vehicle. Ease your car onto the jack stands. Put your car into Neutral. This will allow you to spin the driveshaft using your hands.

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Approximating the Gear Ratio of Your Vehicle

Position yourself under the car. Make sure you have a good view of the inside of the back tires while also allowing you to grab the driveshaft. Check where the differential housing is and the driveshaft next to it. Mark the driveshaft with a paint pen or chalk. Also mark the inside wall of one of the back tires. The idea here is that you should be able to see these “marks” as you rotate the driveshaft with your hands.

Grab the driveshaft and spin it. Count the number of times you were able to spin the driveshaft to complete one rotation of the tire. It would be ideal to have someone watch the tire as it rotates, taking due notice on the number of times the “mark” completed a rotation. Continue spinning the driveshaft and counting the number of rotations the marks on the driveshaft and tire have made.

Use this information to calculate for your car’s gear ratio. For example, if you were able to turn the driveshaft 15 times and saw the tire rotate 5 times, then this is a ratio of 15/5 or 3. The gear ratio of your car is 3:1.

Determining the Actual Gear Ratio of Your Car

If you look at a car’s technical information sheet, gear ratios are never whole numbers. The spinning method does not provide the exact gear ratio of your car. As such, you will have to find the actual numbers by looking for it in the ring gear of the differential.

Get an oil catch pan and place this directly underneath the cover of the differential. Get the appropriate socket and ratchet to remove the bolts on the differential cover. Remove the cover and drain the oil. This may take some time, so be patient.

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Once you have drained the oil in the differential, find the ring gear. It should have numbers engraved or stamped onto its surface. If you cannot see it, try spinning the ring gear by hand. The numbers you see on the ring gear are your car’s gear ratio.

Before you put back the cover of the differential, make sure to clean the mounting gasket first. You can find this on both the housing of the differential and the cover. Ideally, you should discard the old mounting gasket and install a new one.

Replace the cover of the differential and screw in the bolts. Use your torque wrench to tighten the bolts. Make sure to check your service manual to know the specific torque to apply. It is imperative that you adhere to these specifications.

Replace the differential oil that you drained with a new one. Again, check the car manufacturer’s specifications on the correct level of differential oil.

Upon completion of these steps, you can proceed to lower the vehicle. Place your car in Park and engage the parking brake. Lift the back of your vehicle using a floor jack so you can remove the jack stands. Lower your car and remove the chocks at the front wheels.

Calculating the gear ratio of your car is easy. This will help you make the appropriate changes to your car’s setup if you’re planning to modify it or upgrade it.


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