Never really built a gearbox before but knew the basics of a 2 and 4 speed boxes so i smashed them together and build this kinda small 8 speed gearbox : r/legotechnic
Lego 4 Speed Rotary Gearbox with build instructions - YouTube
Sariel.pl » 6-speed RC Servo Transmission
Ultra Compact 8 Speed Sequential Gearbox - LEGO Technic, Mindstorms, Model Team and Scale Modeling - Eurobricks Forums
Shift GEARS at HOME LEGO Engine and 6 Speed (REALISTIC) - YouTube