eli5 why does the cars R gear sound so different to the 1st gear? you can always hear the different when a car is backing up.
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Most of the gears in your car are [helical gears](https://upload.wikimedia.org/wikipedia/commons/e/ed/Anim_engrenages_helicoidaux.gif) where the teeth are tilted. This means that only a tiny part of the tooth it making/breaking contact at any given time and generally multiple teeth are partially engaged at all times. Helical gears run smoother and quieter making them good for things where you want speed and lots of power
The reverse gear in your car is a [straight cut spur gear](https://upload.wikimedia.org/wikipedia/commons/1/14/Gears_animation.gif) and because the teeth are straight a whole tooth makes contact across its entire length at once and later releases across its entire length at once. They’re rougher and noisier because of this since the teeth are never *perfectly* spaced so there’s always going to be some chatter. The straight cut teeth are also cheaper so they’re good for the reverse gear where you don’t need speed, power, or quiet operation because its only occasional so it gives a minor cost savings.
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