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 MOC 8 Speed Sequential Gearbox by Nico71 | Rebrickable - Build with LEGO
Lego's 2018 Bugatti Chiron kit has functioning transmission and 16-cylinder engine