How to Calculate Gear Ratio

People often talk about gearing and gear ratios when discussing sports cars and muscle cars. Learning how to calculate gear ratios can help you tweak the performance of your vehicle to suit your needs.

Gear ratios are necessary for calculating the speed of a gear chain and analyzing the performance of a vehicle. The gear ratio influences the amount of torque transferred from the engine and transmission.

A higher gear ratio helps your vehicle accelerate at a faster speed. A lower gear ratio may improve fuel efficiency by decreasing the engine RPM. You can also change gear ratios to accommodate different tire sizes.

If you want to learn more about gearing, you should first learn to calculate gear ratios. Cargister has you covered!

Steps for Calculating Gear Ratios

Vehicles with rear-wheel drive have two sets of gear ratios to consider: the transmission gear ratios and the differential gear ratios.

Both sets of gear ratios determine how quickly you can accelerate and impact fuel efficiency. Yet, the ratios are determined using different steps.

Transmission Gear Ratios

The transmission uses gears to control the rate of acceleration and to maintain your current speed with greater efficiency. Geared transmissions also help increase the reliability and overall service life of your engine.

Power and torque from the engine are delivered to the front of the transmission, which then rotates the main drive gear (input shaft). The main drive gear then meshes with a cluster of gears.

Most modern cars include six-speed automatic transmissions. With a six-speed transmission, you have six different gears. Each gear produces a different gear ratio.

A higher gear has a smaller diameter while a lower gear has a larger diameter. For example, in first gear, the transmission uses the gear with the largest diameter, which slows the engine’s RPM and torque.

The transmission gear ratio is calculated by dividing the number of teeth on the main drive gear by the number of teeth on the currently driven gear.

For example, the main drive gear may have 45 teeth while the first gear has 15 teeth. This would result in a gear ratio of 3.0:1 (45 divided by 15 equals 3.0). The second gear may have 28 teeth, resulting in a gear ratio of about 1.60:1 (45 divided by 28 equals 1.6071).

First gear has a higher gear ratio, which is needed to get the vehicle moving. With a gear ratio of 3.0:1, the transmission rotates the driveshaft once for every three revolutions of the engine.

As you shift gears to a higher gear, the gear ratio shrinks. With third gear, the transmission often has a gear ratio of 1.0:1, which means that the driveshaft rotates each time that the engine rotates.

Differential Gear Ratios

Along with the transmission, your vehicle may include a rear-wheel differential with a separate set of gear ratios. The differential converts the engine’s torque from the driveshaft to the rear wheels. It also multiplies the engine’s torque and reduces the rotational speed.

The input of the differential that connects to the driveshaft is called the pinion gear. It sits at a 90-degree angle to the ring gear. The pinion gear drives the ring gear, which turns the wheels.

The gear ratio for the differential is determined based on the difference in size between the ring and pinion gears. For example, if the ring gear has 41 teeth and the pinion gear has 10 teeth, the gear ratio is 4.10:1 (41 divided by 10 equals 4.1).

The only way to change the gear ratio of the transmission is to replace the transmission. However, you can replace the ring and pinion gear set.

Changing the differential gear ratio can alter the performance of your vehicle or help compensate for different tire sizes.

For example, if you switch from 30-inch to 35-inch tires, you reduce fuel economy and place more strain on your engine. The newer tires decrease the final drive ratio by 17%. If you previously had a differential gear ratio of 3.56, you may need to switch to a gear ratio of 4.10 to maintain the same performance.


Knowing how to calculate gear ratios is not too difficult. You simply divided the number of teeth on one gear by the number of teeth on the connected gear.

The main sets of gear ratios include the transmission gear ratio and the differential gear ratio. If you want to fine-tune the performance of your vehicle, the differential gear ratio is a good place to start. Increase the ratio to improve fuel economy and decrease the ratio to improve acceleration.

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