LEGO Technic 5+R Speed Transmission Gearbox -car truck NXT EV3 - new parts | eBay
LEGO MOC Sequential 4-speed gearbox by Legonutmeg | Rebrickable - Build with LEGO
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