The passenger car gearbox has, as a rule, for every speed, one gearwheel pair. Only one of the two is solidly and permanently blocked on its shaft. This means that even in neutral, all the gearwheel pairs can mesh with each other. When changing a gear, the free gearwheel of the corresponding pair is connected with its shaft. During the gear change the power flow must be interrupted. Before the transmission of power, the differing RPMs are adjusted by cone clutches running in oil. The actual gear change activity can occur mechanically, hydraulically, pneumatically or electrically.
Gear lever almost always connected to a shifting fork
Almost all slide feed or sliding clutch gearboxes have helical gearing. Of the several gearwheel pairs, in neutral gear, the driving gearwheel is connected either directly or through a gearwheel pair with the crankshaft, the driven gear is connected to the axle drive. One wheel is splinted to a shaft, the other of the pair, turns freely on another shaft. If a gear is engaged, one of the gearwheel pairs is moved into the power flow. A torque proof switch- or sliding sleeve is shifted, linking the free running gearwheel with its shaft.
1st gear - high torque, top gear - high speed
The passenger car and vans use the first three gears for slow, and fifth and possibly sixth gear for higher speeds. If one disregards the reverse gear, the transmission ratio is highest in the first gear. It is 1, if input shaft and output shaft have the same RPM. This is mostly the case in 4th gear with coaxial gearboxes.
The higher the torque, the larger and broader the cog
Should the gearbox have to transfer more torque, broader gearwheels are used and the shafts and bearings are strengthened. Although, (except for Fuller gearboxes) in principle no differently constructed, gearboxes for heavy trucks, because of the torque to be transferred, require substantially more space. A gearwheel gearbox (without torque converter) still proves to be the most effective way to transmit power. With not too much helical gearing, 95%, and possibly even more efficiency can be expected. 09/08
In general ...
... the gears R and 1 - 3, decrease the input speed.