Jack
It is just whichever stat is "furthest from optimal" at the moment. What's the issue with the current behaviour?
Previous to the 2025 update if you trained physical stats drivers would lose weight + gain stamina. I don’t remember if it happened when training mental/driving ability too, but it made a lot of sense. Since the february 2025 update this was removed and they only get either stamina or lose weight, not both.
May bringing this back be in your interest? Right now you waste a ton of time trying to get drivers in shape, while other stats get worse because of atrophy. In my opinion it makes a lot of sense to lose weight, regardless of what you are training.