The more experience points Bob gets the faster his stats will rise, it's not about the car.
When you get to the higher B spec levels and introduce a new Bob, you can throw him into a high level race, even though he is level 0.
Some high level races will have 15,000 to 300,000+ points, when the race finishes his stats, and his level, will shoot up to anything from level 10 to 20+. His skill and strength will shoot up the same way too.
Whereas your first Bob, who trawels through Bspec from level 0 to 1 to 2 etc etc will have his stats go up very very slowly.