I run a gearbox for 2 races, and always use an old gearbox for testing days. I'm pushing most gearboxes past 1000km, but wouldn't use one that had done more than 400km in a race.
Changing boxes every race, and STILL failed less than half its design distance. I wasn't even pushing. What's the point of a racing game that doesn't let you RACE!
I literally don't know what I'm supposed to do. I designed a new gearbox and it had even worse performance than the original.
You designed a shit gearbox, kept racing it, and asking why it failed?
Design another, see how that goes. Not every new design will be better, some are worse, it's random. If it's crap, the only way to better it is redesign or improve it further by refining