Gear Ratios

Part 3 of 6    4/17/17

Inside the transmission are a series of variously sized, toothed gears that produce torque. Because the gears that interact with each other are different sizes, torque can be increased or decreased without changing the speed of the engine’s rotational power all that much. This is thanks to gear ratios.

Gear ratios represent the gears’ relation to each other in size. When different sized gears mesh together, they can spin at different speeds     and deliver different amounts of power.

Let’s look at a dumb-downed version of gears in action to explain this.    Say you have an input gear with 10 teeth (by input gear, I mean a gear  that is generating the power) connected to a larger output with 20 teeth     (by output gear, I mean a gear that is receiving the power). To spin that   20-toothed gear once, the 10-toothed gear needs to turn twice because   it’s half as big as the 20-toothed gear. This means that even though the  10-toothed gear is spinning fast, the 20-toothed gear is turning slowly.   And even though the 20-toothed gear is turning more slowly, it’s    delivering more force, or power, because it’s larger. The ratio in this arrangement is 1:2. This is a low gear ratio.

Or let’s say the two gears connected to each other are the same size      (10 teeth and 10 teeth). They’d both spin at the same speed, and they’d both deliver the same amount of power. The gear ratio here is 1:1. This      is called a “direct drive” ratio because the two gears are transferring the  same amount of power.

Or let’s say the input gear was larger (20 teeth) and the output gear was smaller (10 teeth). To spin the 10-toothed gear once, the 20-toothed gear would only need to turn half way. This means that even though the 20-toothed input gear is spinning slowly and with more force, the 10-toothed output gear is spinning fast, and delivering less power. The gear ratio here is 2:1. This is called high gear ratio.

Let’s bring that concept back to the purpose of the transmission.

First Gear. It’s the largest gear in the transmission and enmeshed with a small gear. A typical gear ratio when a car is in first gear is 3.166:1. When first gear is engaged, low speed, but high power is delivered. This gear ratio is great for starting your car from a standstill.

Second Gear. The second gear is slightly smaller than first gear, but still   is enmeshed with a smaller gear. A typical gear ratio is 1.882:1. Speed is increased and power decreased slightly.

Third Gear. Third gear is slightly smaller than the second, but still enmeshed with a smaller gear. A typical gear ratio is 1.296:1.

Fourth Gear. Fourth gear is slightly smaller than the third. In many vehicles, by the time a car is in fourth gear, the output shaft is moving at the same speed as the input shaft. This arrangement is called “direct drive.” A typical gear ratio is 0.972:1

Fifth Gear. In vehicles with a fifth gear (also called “overdrive”), it is connected to a gear that’s significantly larger. This allows the fifth gear to spin much faster than the gear that’s delivering power. A typical gear ratio is 0.78:1.

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