My brain cant seem to grasp the concept behind this sorry

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They don’t need to be higher unless you want to go the same speed in a gear lower as before. The “gear” is just the ratio of your engine’s shaft spinning rate versus your wheel spinning rate. The higher the gear, the closer they are to 1:1, the lower the gear, the more force the engine can exert for a given RPM in exchange for speed.

A smaller gear needs to rotate further to cover the same distance (on its outside edge) than a larger gear does. So in order to make the wheels of your car rotate at the same speed, a lower gear needs to make more rotations per minute, compared to a higher gear.

It is an identical process to riding a multi-gear bicycle if you’ve done that — at lower gears it’s much easier to pedal, but you need to pedal much faster to compensate. At higher gears you have to push harder, but each cycle of the pedals gets you more speed.

It works the same way as riding a bike. When you need to go up a hill, you switch the chain to low gear, and you pedal faster to go the same speed but it’s easier on you. The gear ratio is low, because it takes more turns with your feet (numerator) to complete a full turn of the back wheel (denominator) than it would if you were in high gear.

Low gear means fewer rotations of the wheels for a rotation of the engine, which also means more rotations of the engine for a rotation of the wheels.

High gear means more rotations of the wheels for a rotation of the engine, which also means fewer rotations on the engine for a rotation of the wheels.

Rotation of the wheels is a multiple of your traveling speed because the wheels are rolling with the road.

The tachometer (thing showing rpm) is measuring rotations of the engine, so at a set speed (set wheel rotations) the numbers (rotations of the engine) will be higher in low gear.

Engine RPM * Gear Ratio = Wheel RPM

That’s how it works. When we say a “low” gear, the number for the gear ratio is low. A “high” gear has higher number for the ratio. Most people don’t know their exact ratios, but it doesn’t matter. We don’t know wheel speed in RPM either. What matters is the relationship, and how the multiplication works.

If the gear ratio goes down, but wheel speed remains the same, then engine speed must go up to compensate and keep the equation equal. If you don’t, something gives… the engine speeds up and/or the car jerks to slow down.