Manual (standard) transmissions, like all other mechanical mechanisms, have evolved from simpler to more technically complex devices. The manual transmission was the only option available in motor vehicles for many years, but the automatic transmission is slowly supplanting it. While a number of automatic transmissions have surpassed the manual transmission in terms of gas mileage and vehicle control, a variety of manual transmission types are still available to accommodate different needs and driving styles.
Primary manual transmission types
Manual transmissions fall into two basic types. The first is the sliding gear type, which is now obsolete. This transmission only had two moving parts that were rotating while the transmission was in neutral gear: the main drive gear and the cluster gear. Once the clutch was depressed, the driver could move the gear shifter, which in turn caused the shift linkage and forks to move a gear on the main shaft above the cluster gear. Upon release of the clutch, the gears would mesh, and the power from the engine went to the drive wheel or wheels. Because the main shaft had multiple gears of different sizes and with different tooth counts rotating at varying speeds, a gear clash might occur. This would result in the inability of the driver to shift gears to increase or even maintain speed.
The constant mesh is the second manual transmission type. It accounts for all modern manual transmissions. With the constant mesh transmission, the main shaft gear and the cluster gear are always "meshed" to one another. The main shaft gear, the cluster gear and the main drive gear are in constant motion when the transmission is in neutral. Constant mesh manual transmissions have synchronizers that prevent gear clashing or grinding, unlike their sliding gear predecessors.
Varieties in manual transmissions
Vehicles manufactured in the 1960s usually had manual transmissions with four forward speeds, one reverse gear and no overdrive. Overdrive is a transmission gearing setup that allows for the drive shaft going to the wheels to rotate at higher revolutions than the crankshaft of the engine. This allows for substantially improved gas mileage and considerably less wear and tear on the engine without a loss of speed. This type evolved into four-speed manual transmissions with overdrive, five-speed manual transmissions with overdrive and, ultimately, six-speed manual transmissions with overdrive.
Manual transmissions are still predominant in the commercial trucking industry. Due to large and extremely heavy loads, using varying gearing scenarios can accommodate the work while maximizing fuel efficiency. Speed and torque are inversely proportional, so using an automatic transmission in those vehicles would result in lower gas efficiency, more frequent mechanical breakdowns and greater cost to transportation companies. This would also lead to increased costs to the end-point consumer of goods transported by truck.