Detailed on this page is a simplified explanation of a car’s gear ratios based on a typical 5 speed manual gearbox.
Car Gear Ratios Explained
A car’s engine operates within a certain range of speed. If a car didn’t have gears, you would only be able to move at the speed of the engine. This would make moving off from a standstill very difficult and without gears, the top speed would be slower.
Different size cogs inside the gearbox provide different gear ratios. As an example, in a gearbox with 5 forward gears, all gears (1 to 5) operate on the same principle; a cog is connected to the engine side called the input gear and a cog connected to the road wheels side is called the output gear.
For 1st gear, the input gear is much smaller than the output gear and this provides the gear with the highest ratio. With this ratio, the engine rotates many more times compared to the road wheels and this provides the highest amount of power (torque); ideal for moving off from a standstill but not very good for speed.
5th gear is the opposite; less pulling power (torque) is required because the car has momentum and so we trade torque for speed. The input gear is larger than the output gear and this provides the lowest gear ratio.
How Are Gear Ratios Worked Out?
Let’s look at the gear ratio of a 5 speed manual gearbox. Gear ratios aren’t usually quite as exact as these, but it makes understanding it a little easier.
Gear ratios are worked out by dividing the number of teeth on the input gear (or cog), by the number of teeth on the output gear. On low gears such as 1st and 2nd, the output gears are larger and have more teeth on them when compared to the input gear that they’re meshed to, meaning the output gears rotate slower.
As we know, the input gear is driven by the engine and the output gear determines the speed in which the road wheels rotate. So based on the gear ratios above:
In 1st gear, the engine makes 3.00 revolutions for every one revolution of the transmission’s output gear. This is because the output gear is bigger and has 3 times the amount of teeth than the input gear.
In 2nd gear, the engine makes 2.00 revolutions for every one revolution of the transmission’s output gear. This is because the output gear is bigger and has 2 times the amount of teeth than the input gear.
In 4th gear, the gear ratio of 1:1 and with the input and output gears being the same size, it means that the engine and the transmission’s output rotate at the same speed. Both input and output gears have the same amount of teeth.
In 5th gear, the engine’s revolutions are half that of the output gear. The output gear is smaller and is rotating faster than the input gear – ideal for high speed. The output gear is smaller and has fewer teeth than the input gear.