The absolute bread and butter of your Gearbox, the Gears are an incredibly important part of your AEG's operation and need to be effective for smooth firing. The better made your Gears are, the longer your AEG is going to last, it's a simple as that. However, massively up-rated Steel Gears are pretty useless if you're still using a Polymer-toothed Piston, so be sure to up-rate the rest of your Gearbox as you improve vital parts. Gear sets are rated by a 'Ratio' value which is essentially how many turns the motor needs to make for a complete cycle. There are plenty of 'Gear Ratio Calculators' available online to help you work out exactly what you need, but a rule of thumb is a smaller ratio will result in a higher rate of fire, at the cost of more work for the motor.