As in a-spec, which car you pick doesn't change the xp you can earn from the race. Only your finishing position is a factor.
B-spec Bobs use the same xp leveling system as you do. However the way they get xp is different.
If you enter a race where you can have more than 1 bob, the amount of xp on offer to the bobs (not yourself) is multiplied by the total number of bobs allowed in the race. 4 bobs allowed, 4 times the amount of xp, which will be shared between them. So if you enter 1 bob, you get the standard amount, but he will get 4x that amount. If you enter 2 bobs, you get the standard amount, but they will share the 4x amount between themselves. However it gets slightly more complex as the bobs will share the xp based either on the number of laps they did/time spent driving, not entirely sure which.
