What is the transmission ratio of the main reducer?

The transmission ratio of the main reducer, also known as the final drive ratio, is the fixed gear reduction between the driveshaft and the driven wheels. It determines how many times the driveshaft must rotate to turn the wheels once. Common ratios range between 2.5:1 and 4.5:1 for most passenger cars.

A lower ratio (e.g., 2.5:1) improves fuel efficiency and reduces engine RPM at motorway speeds, making it ideal for long-distance cruising in vehicles like the Ford Focus. A higher ratio (e.g., 4.5:1) provides better acceleration and towing capacity, often found in performance cars or commercial vehicles.

The ratio is calculated by dividing the number of teeth on the ring gear by the number on the pinion gear. For example, a ring gear with 40 teeth and a pinion with 10 teeth yields a 4:1 ratio.

Modern vehicles with adaptive transmissions or electric drivetrains (e.g., Tesla Model 3) may use software to simulate different ratios, but traditional ICE cars rely on fixed mechanical gearing.

From an engineering perspective, the main reducer’s transmission ratio is a critical design parameter balancing performance and efficiency. It directly affects torque delivery, top speed, and fuel consumption. For instance, a BMW iX with a 3.2:1 ratio prioritises smooth high-speed cruising, while a Vauxhall Corsa might use a 3.8:1 ratio for urban agility.

The calculation formula for the transmission ratio of the main reducer is:

Transmission ratio = applied torque ÷ 9550 ÷ motor power × motor power input speed ÷ service factor.

The principle of the reducer's transmission ratio is that the ratio of the instantaneous input speed to the output speed in a mechanism is called the transmission ratio of the mechanism. The reducer refers to the reducer on the drive axle and is not the same device as the commonly referred-to gearbox. The total transmission ratio of a vehicle is the product of the gearbox's transmission ratio and the main reducer's transmission ratio. The main reducer functions in any forward gear or reverse gear of the gearbox. For common vehicles, the transmission ratio of the main reducer is a fixed value.

An analysis of the reducer's transmission ratio is as follows:

The transmission ratio is the ratio of the number of teeth engaged between the next-level gear and the previous-level gear;

A transmission ratio less than 1 means a large gear drives a small gear, resulting in acceleration;

A transmission ratio greater than 1 means a small gear drives a large gear, resulting in deceleration.

There are two types of transmission ratios in a vehicle's transmission system: the speed ratio of the main reducer and the speed ratio of the gearbox. In the same vehicle model, the speed ratio of the main reducer is a fixed value, while the speed ratio of the gearbox varies with the gear used.