Driver experience is raised when you train them, the overall stat is your Driver's attributes divided by 20, which is considered the average. It is not agree'd that Health and weight are included in the overall.
Every race drops each attribute by 0.1, which would equal to a 1.8 drop (as experience and talent do not lower).
Training any stat excluding physical will increase that net attribute by 3.0
So generally every race, you gain 1.2 /20 overall.
It would take about 14 races to raise your driver's overall level by 1.
