Final answer:
To determine when to shift gears in a manual or semi-automatic transmission, you monitor the engine's RPM and shift up around 2,000 to 3,000 RPM for smooth acceleration, or down for more power or engine braking. Modern vehicle transmissions improve fuel economy by maintaining the engine speed optimally.
Step-by-step explanation:
When driving a car with a manual or semi-automatic transmission, you need to shift gears based on the engine's speed (RPM) and the driving conditions. For a smooth and efficient driving experience, it is essential to know when to shift to a higher or lower gear. Typically, you shift up to a higher gear when the engine reaches a certain RPM, which varies among vehicles but is commonly around 2,000 to 3,000 RPM for smooth acceleration.
You would shift down when you need more power from the engine, such as when going uphill, or when slowing down to use engine braking effectively. Modern vehicles are equipped with various transmission technologies like 7+ speeds, dual-clutch transmissions (DCTs), and continuously variable transmissions (CVTs) which greatly enhance fuel economy and reduce CO₂ emissions by maintaining optimal engine speed.