My brain cant seem to grasp the concept behind this sorry
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The tachometer, which is what displays RPM’s, is showing you the engine speed. Engines have a fairly limited range of speed they can operatate so a transmission is used to convert the narrow range of engine speeds to a wide range of wheel speeds. This is done by have several different sets of gears that have different gear ratios. Gears are called low or high because of the vehicle speed they are intended for not because of the engine speed.
A gear ratio is input speed/output speed. First gear in a car may have a ratio of approximately 5:1, which means for every five revolutions of the input gear (connected to the engine), the output gear (connected to the wheels) rotates once. However, torque is inversely proportional to speed, meaning the output while turning slower will be able to apply more force to accelerate the vehicle
When you want to accelerate you need to apply more force to the road via the tires so you want more torque so a high gear ratio is used to convert high engine speed into lower wheel speed with higher torque. As you shift to higher gears the gear ratio decreases, so the torque available to accelerate you decreases but the vehicle can travel faster without over spinning the engine. As you reach cruising speed you need a lot less torque because now you just need to overcome air resistance and friction to maintain speed so cars can shift to a higher gear with an even lower gear ratio.
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