What is the final drive ratio?
4 Answers
The calculation formula for the final drive ratio is: Drive ratio = applied torque ÷ 9550 ÷ motor power × motor power input speed ÷ service factor. The principle of the reducer's drive ratio is that the ratio of instantaneous input speed to output speed in a mechanism is called the drive ratio. The reducer refers to the one on the drive axle and is not the same device as the commonly mentioned transmission. The total drive ratio of a vehicle is the product of the transmission ratio and the final drive ratio. The final drive is effective in any forward gear or reverse gear of the transmission. For common vehicles, the final drive ratio is a fixed value. The analysis of the reducer's drive ratio is as follows: The drive ratio is the tooth count ratio between the next gear level and the previous gear level when they mesh; A drive ratio less than 1 means a large gear drives a small gear for acceleration; A drive ratio greater than 1 means a small gear drives a large gear for deceleration. There are two types of drive ratios in a vehicle's drivetrain: the final drive ratio and the transmission ratio. In the same vehicle model, the final drive ratio is a fixed value, while the transmission ratio varies depending on the gear used.
I've been driving a taxi for ten years and often think about the final drive ratio. It's like the gear ratio on a bicycle, directly determining whether the car has more power or higher speed. For regular family cars, it's usually between 3.5 and 4.2. If you're driving an SUV or pickup truck, this number can go above 4.5 to provide enough strength for climbing hills or hauling loads. Choose a smaller ratio for better fuel efficiency or a larger one for quicker acceleration. Last time I drove an old Jetta with the final drive ratio increased by 0.3, the take-off was noticeably faster, but the tachometer needle went much higher on the highway. You have to weigh the pros and cons yourself.
Back when I was learning auto repair, my mentor always stressed that the final drive ratio shouldn't be tampered with casually. The transmission and engine matching is like a married couple living together, where the final drive ratio acts as the matchmaker coordinating their relationship. Compact cars generally use a ratio around 3.8, while rear-wheel-drive vehicles tend to have higher ratios. According to technical manuals, the BMW 3 Series Sport version can go as low as 3.15, whereas the Highlander reaches up to 4.3. Modifying this ratio requires altering the differential gear set, and if done improperly, it can cause driveshaft wear. Once, while helping a customer diagnose abnormal noises, we discovered they had installed an oversized gear set with a ratio exceeding 4.5, which had vibrated so severely it cracked the axle shaft.
The most thrilling part of modding is adjusting the final drive ratio. My Civic's stock 4.3 was way too conservative, so I directly swapped in a 4.7 final drive gear. The 0-100km/h time in third gear shortened by 0.8 seconds, and the acceleration push feels like I've changed the engine. But don't get greedy - anything above 5.0 makes the fuel consumption skyrocket. There's a guy in the MX-5 club who went up to 4.9 - it's indeed fierce on track corners, but for daily driving he's always riding the clutch, needing new clutch plates every three months.
