system
Given the following post, respond as if you're commenting on Reddit:
human
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.
gpt
Straight from Google:

Reverse gears are straight cut while forward gears are helical (a helix or spiral shape) because it creates a bigger contact area for the gears which creates higher torque. 

You don't need a lot of torque when reversing so the gear is cut straight creating less contact area which makes a different sound then the forward gears.

ElI5: Smooth gear makes different sound than a gear with grooves.