Hi! I created an adaptor for mijael08 gearbox (link below) to make it automatic ( robotic automated gearbox) and I installed it in a RC car with bluetooth controls.. It has 4 gears + reverse + neutral, steering and I programmed it to act like a real car, it revs faster and goes slower to idle and steering is adaptable and is slowly steering to the centre, like a real car. The gearbox does not need to be printed with all the parts, only the gears, shafts and mounting plates are enough for our setup. It uses 6 bolts to mount the adaptor on to the gearbox ( 2 holes need to be drilled ). It uses 3 MG995 servomotors with levers to change gears. The mechanism uses 3 levers to change between gears, 1 lever for gear 1-2, a longer lever for gear 3-4 and a lever for reverse gear. The levers are driven by 3 MG995 servomotors. I recommend to use minimum 30% infill on the gears or more border layers because of the wear. When shifting gears, the gearbox grinds a little bit until it fully connected. Using silicone vaseline makes it run way better. I used a 12V 133RPM DC motor to turn the gearbox, but for a demo a 6v dc motor is strong enough. My rc car si almost 3 kg and I needed a stronger motor. I calibrated my servos before installing so they turn +30 and -30 degrees to connect to a gear. I will create a github project to show you the code I wrote, the gearbox has to be safe all the time ( only 1 gear connected at a time) otherwise the shaft will twist and weaken.

Short video: https://youtube.com/shorts/UJGhuDALUyE?si=x4nDl_AYz0sBbfoA

Parts used that were created by other creators:
gearbox: https://www.thingiverse.com/thing:3726483
steering: https://www.thingiverse.com/thing:3239159
V8 engine: https://www.thingiverse.com/thing:1195361
Differential: https://www.thingiverse.com/thing:11836

Every part used has been modified to adapt to my needs. If you are interested in my project, leave me a message. I would be more than happy to help someone build it better than I did.