How does a manual transmission change the way a car pulls?

A car engine works over a limited range of rotation speeds, while the wheels must turn slowly when the car starts and faster as it travels. A transmission connects these different needs. It provides several gear ratios between its input and output shafts, changing how quickly the output turns for each turn of the input.

In a typical manual transmission, several gear pairs remain meshed with one another. Some gears can spin freely around the output shaft until selected. Moving the gear lever shifts a mechanism that locks the chosen gear to that shaft. The other gears may still spin, but they do not all force the output to follow their different speeds. A synchronizer helps match speeds before the selected connection locks.

A low gear has a large reduction ratio. In the ideal illustration, an input turning at 3000 revolutions per minute through a 4:1 reduction produces 750 rpm at the output. The same example multiplies an input torque of 40 newton-meters to 160 newton-meters. Torque describes a turning effect. The stronger turning effect comes with fewer output turns each second.

Selecting a 1:1 ratio instead gives 3000 rpm and 40 newton-meters at the output. An ideal gear train passes the same power in either setting: multiplying torque requires a matching reduction in speed. Real transmissions lose some energy as heat. The gears change the relationship between speed and torque; they do not create the engine's energy.