A-Spec poitns are factored by:
Weight
Power
Suspension
Tires
So when you start modding those, the points get lower, Tires and suspension take away more aspect poitns, while heavier cars give more.
The system is slightly flawed, because the calculations can be fooled. for example, get a dodge ram, and mod it to hell, then take it to a race, you'll get 180+ points easy each race, no challenge.
B-Spec points - Best way to build these up are the family cups on the Nurb. Easy you'll get around 500+ points, lol. It levels up your driver crazy quick. Just remember, start him out with lower power cars, then start to put him in with high powered cars, he'll be winning races like there's no tomorrow.