A few words about gear ratios
Cars use gear ratios in both the drive axle and the transmission to amplify engine power. You cannot change the gear ratio of vehicles with front-wheel drive systems; only in rear wheel drive.
There are two main components to any powertrain. These include the drive gear and the driven gear. The drive gear is the component that receives the rotational force. In automobiles, the rotational force comes from the engine. The drive gear causes the other driven gear to rotate. Some mechanics refer to them as the input gear and the output gear, respectively.
So what is the gear ratio? This is the relationship in rotational speed rates between the input gear (drive) and the output gear (driven). For example, a simple 2-speed gear train that has a 2: 1 ratio means that the drive gear will have to turn twice for the driven gear to make a full rotation.
When applied to cars, the gear ratio is the ratio of the pinion to the crown. This is the equivalent of the input and output gears, respectively.
Calculate gear ratios: how to calculate gear ratio
Calculating the ratio of any given gear train is pretty easy. All gears have teeth. They interlock with the teeth of the adjacent gear, allowing it to rotate in the opposite direction to the rotation of the drive gear.
gear ratio calculator
When calculating the gear ratio, you only need to determine the number of teeth of both gears. For example, let’s say you have a first gear or input gear that has 20 teeth and a second or output gear that has 30 teeth. Divide the number of teeth on the output gear by the number of teeth on 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 to be able to turn the driven gear once.
Unfortunately, the gears in our cars are not that straightforward. So the next question people often ask is how to find the gear ratio in more complex drivetrains. To do this, you can still follow the same formula when calculating the ratio of more complex gear trains. For example, a 3-speed system means you have an idler gear between the drive gear and the driven gear. Intermediate gears are often included to allow the driver and drive wheels to turn in the same direction. Idlers or gears can also change the plane of rotation of the gear.
how the transmission ratio is calculated
Let’s say the first gear has 7 teeth, the idler gear has 20 teeth, and the driven gear has 30 teeth. There are two ways to calculate the ratio of this gear train. First, divide the number of teeth on the driven gear (n = 30) by the number of teeth on the drive gear (n = 7). This will result in a 4.285 or 4.3: 1 ratio. This means that the input gear will have to turn 4.3 times to allow the output gear to make a full rotation.
The second method involves calculating the gear ratios of the intermediate gears. This means that you have 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 produce 4.3.
The result remains the same as in the first method, which is 4.3. The nice thing about the second method is that you can also get an idea of the relationship between the individual gears. This will also give you an idea of why your transmission has different gear ratios as you go through gears.
For example, a General Motors TH-200-4R automatic transmission might have a gear ratio of 2.74 in first gear, 1.57 in second, 1.00 in third, and 0.67 in overdrive. If you notice it, the …