In NASCAR, all of the race cars have manual transmissions. They use a four-speed manual transmission called the Andrews A431 Transmission.
The transmission is the part of the car that transfers power and torque from the engine to the rear axle. The car's transmission allows the gear ratio between the wheels and the engine to adjust according to the car's speed. Manual transmissions require the driver to choose the correct gear and engage the clutch pedal, while automatic transmissions simply rely on a torque converter to change gears. Because NASCAR cars use manual transmissions, they require the driver to shift gears when speeding up or slowing down.
NASCAR transmissions make use of an H-pattern gearbox. This setup requires the driver to use their right hand to grab the stick and push it into a particular gear. Unlike the transmissions on normal manual cars, NASCAR cars do not require the driver to press the clutch pedal while shifting gears. Although NASCAR cars have clutch pedals, these are rarely used when shifting gears.
Instead, drivers shift gears by matching the car's speed to the car's RPM (revolutions per minute). This process is called rev matching and requires the driver to have a great sense of the speed at which the gear should be shifted. This is challenging because there are no RPM gauges in NASCAR cars. Shifting at the wrong time or shifting into the wrong gear can cause damage to the car's transmission.
All NASCAR transmissions have straight cut gears, meaning the teeth of the gear face outwards as opposed to the helix shape of gears on normal cars. Straight cut gears are essential for racing vehicles because they produce no axial load and increase torque, which allows the car to perform more efficiently. Because the axial load is decreased by the straight cut gears, it is easier for drivers to shift gears without having to use the clutch pedal. Due to the fact that straight cut gears mesh together all at once, a shrill noise is produced in the transmission that can be heard while watching NASCAR races. NASCAR transmission development is heavily regulated to create an equal playing field for the drivers. No parts of the transmission may be modified without prior approval from the governing body.
It is expected that NASCAR will make the switch to a manual sequential transmission in 2021. Reports indicate that NASCAR's next-generation stock car features a six-speed gearbox, as opposed to the four-speed gearboxes currently used. NASCAR has relied on the four-speed H-pattern gearbox since the sport's inception in 1949, so the upgrade to a sequential six-speed transmission in 2021 is significant.
Unlike the H-pattern gearbox that NASCAR has traditionally used in cars, sequential transmissions make use of a lever that is tapped forward and backward to change gears. This setup allows the driver to shift up or down in gear, but does not require the driver to manually find the gear they are looking for.