Little info about B-Spec, might be useful to some: every B-Spec driver you have has a bar in his stats called "career". This bar is filled through the number of races this driver participates in, not the number of driven miles.
When this bar is full, the driver will become slower and lose some of his talents, because his career is over. It appears he will still be choosable as a driver, but he will probably be so bad that you will have to remove him from your team and start with a fresh one. So when you train your B-Spec guys for a specific event, do not only pay attention to their experience, but to their career as well. If you fill their experience bars by letting them do an endless number of small races, their career might be over before they become useful to you.