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Gear size refers to the geometric characteristics of gears calculated through core parameters such as module, number of teeth, and pressure angle, which essentially reflect the physical specifications and meshing capability of gears. As a fundamental parameter, the module (m) is defined as the ratio of the pitch to the circumference π (m = p/π), directly determining the tooth height and tooth thickness. For example, the tooth height of a gear with a module of 0.5 is 1.125 mm (2.25 × 0.5), while that of a gear with a module of 1.0 doubles to 2.25 mm. Gear size calculation covers key data such as reference circle diameter (d = mz) and addendum circle diameter (da = d + 2m). For instance, the reference circle diameter of a gear with 16 teeth and a module of 0.4 is 6.4 mm. It should be noted that gears that mesh with each other must have the same module; otherwise, normal transmission cannot be achieved. The standardized design of gear sizes (such as the module series specified in JIS B 1701) ensures manufacturing compatibility, while the modified gear technology can adjust the center distance to meet non-standard requirements. These parameters collectively affect the torque transmission efficiency and mechanical strength of gears, and are core considerations in the design of transmission systems such as automotive gearboxes.

The gear ratios from 1st to 7th gear in an automobile transmission refer to the rotational speed ratio between the input shaft and the output shaft at each gear position. Their numerical design directly affects the vehicle's power output and fuel efficiency. Taking manual transmissions as an example, the 1st gear ratio is usually between 3.0:1 and 5.0:1, achieving high torque output through a large gear ratio, which is suitable for starting or climbing. As the gear position increases, the gear ratio gradually decreases. For instance, the 5th gear ratio is approximately 0.7:1 to 1.0:1, while the 6th or 7th gear (more common in high-performance or energy-efficient models) may further drop to around 0.6:1 to reduce the engine speed during high-speed cruising. The gear ratio distribution logic of automatic transmissions is similar, but the specific values vary due to differences in brand technologies. For example, the 1st gear ratio of some 7-speed dual-clutch transmissions is about 4.7:1, and the 7th gear ratio may be 0.6:1. It should be noted that the actual gear ratio is comprehensively influenced by the vehicle's positioning, engine characteristics, and final drive ratio. For example, the low gear ratios of commercial vehicles may be as high as 6:1 or more to meet heavy-load requirements, while the overdrive gear is designed to improve fuel economy through a gear ratio of less than 1. It is recommended to consult the technical manual of the specific vehicle model to obtain accurate data.

D4 is a gear identifier for automatic transmission vehicles, indicating that the transmission can automatically shift between gears 1 and 4, making it suitable for most daily driving scenarios. During normal driving, the system automatically selects the appropriate gear based on vehicle speed, engine RPM, and road conditions. For instance, it starts in first gear and progressively shifts up to fourth gear as speed increases to optimize power delivery and fuel efficiency. Common automatic transmission gear positions include P (Park), R (Reverse), N (Neutral), and D (Drive). Within the Drive mode, sub-modes like D3 restrict the transmission to third gear maximum, which is ideal for hill climbing or overtaking, while D4 is better suited for steady-state driving conditions such as highway cruising. In certain vehicle models, D4 may also refer to engine technology specifications—Toyota's D4-series engines, for example, employ direct fuel injection—though this interpretation depends on specific model context. While proper use of D4 enhances driving smoothness, switching to lower gears is recommended in challenging conditions (e.g., steep inclines or heavy traffic) to maintain better vehicle control.

The numbers 1, 2, and 3 on a car's gear shift typically appear in manual transmission vehicles or the manual mode of automatic transmissions, each representing different gear ratios and power output ranges. Gear 1 is the starting gear, providing maximum torque, suitable for moving the vehicle from a standstill or climbing steep slopes. Gear 2 is used for low-speed driving or gentle inclines, serving as a transition between Gear 1 and Gear 3. Gear 3 is suitable for medium-speed driving, such as urban roads or situations with speeds between 40 to 60 kilometers per hour. These numbered gears adjust the relationship between engine speed and wheel speed to achieve more efficient power delivery and fuel economy. For automatic transmission vehicles with manual mode (e.g., M or S mode), drivers can manually select gears 1 through 3 to handle specific road conditions, such as using lower gears during long descents to employ engine braking and reduce brake system strain. Proper understanding and use of these numbered gears can improve driving safety while optimizing vehicle performance.

There are three main methods for calculating the gear ratio. The first is based on the number of gear teeth, with the formula: gear ratio = number of teeth of the driven gear ÷ number of teeth of the driving gear. For example, if the driving gear has 20 teeth and the driven gear has 40 teeth, the gear ratio is 2:1, meaning that for every full rotation of the driving gear, the driven gear rotates half a turn. The second method uses the inverse relationship of rotational speeds, with the formula: i = rotational speed of the driving gear ÷ rotational speed of the driven gear = number of teeth of the driven gear ÷ number of teeth of the driving gear. If the driving gear rotates at 3000 rpm and the driven gear at 1500 rpm, the gear ratio is also 2:1. The third method combines torque and power parameters, with the formula: transmission ratio = operating torque ÷ (9550 ÷ motor power) × motor input speed ÷ service factor, which requires integrating motor performance and operating condition data. Gear ratio design directly affects vehicle performance. A large gear ratio (e.g., 1st gear) is suitable for climbing or heavy loads, as it increases torque but sacrifices speed. A small gear ratio (e.g., 5th gear) is beneficial for high-speed cruising and reducing fuel consumption. Modern transmissions optimize shift smoothness and fuel efficiency through multi-speed dense gear ratios (e.g., 8-speed transmissions). Additionally, electronic gear ratio technology can improve control precision by adjusting the pulse equivalent—for instance, optimizing the pulse equivalent from 2.44 μm/pulse to 1 μm/pulse can significantly enhance machining accuracy. A reasonable gear ratio configuration needs to balance power output, fuel economy, and driving comfort.