The Goal of a Transmission
Cars need transmissions because of the physics of the gasoline engine. First, any engine has a redline -- a maximum rpm value above which the engine cannot go without exploding. Second, if you have read about Horsepower, then you know that engines have narrow rpm ranges where horsepower and torque are at their maximum. For example, an engine might produce its maximum horsepower at 5,500 rpm. The transmission allows the gear ratio between the engine and the drive wheels to change as the car speeds up and slows down. You shift gears so the engine can stay below the redline and near the rpm band of its best performance.

Ideally, the transmission would be so flexible in its ratios that the engine could always run at its single, best-performance rpm value. That is the idea behind the continuously variable transmission (CVT). A CVT has a nearly infinite range of gear ratios. In the past, CVTs could not compete with four-speed and five-speed transmissions in terms of cost, size and reliability, so you didn't see them in production automobiles. These days, improvements in design have made CVTs more common. The Toyota Prius is a hybrid car that uses a CVT.


The transmission is connected to the engine through the clutch. The input shaft of the transmission therefore turns at the same rpm as the engine. A five-speed transmission applies one of five different gear ratios to the input shaft to produce a different rpm value at the output shaft. Here are some typical gear ratios:

Gear
Ratio
RPM at Transmission Output Shaft
with Engine at 3,000 rpm
1st 2.315:1
1,295
2nd 1.568:1
1,913
3rd 1.195:1
2,510
4th 1.000:1
3,000
5th 0.915:1
3,278

 

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BACK TO CONTENTS
Introduction to Manual Transmissions
The Goal of a Transmission
A Very Simple Transmission
A Real Transmission
Synchronizers