Here is my solution:
You need at least 50 Carpoints. 1 Point is earned with 50000 Skillpoints.
how to get them as fast as possible?
1. Grab Ken Blocks Hoonigan Focus
2. I made some tuning adjustments for this car to make it even more aggressive and faster. just search for my gamertag and the "xp grind" tuning.
3. start to unlock every perk you can with this car. You can grab the "at one with the car" Achievemt too.
4. you can go berserk with this car at anyplace and any time. go to the windmill park and hit the throttle.
as soon as you unlock most of the skill perks you will see why i suggest that car. it is a xp machine. during 2 forzathon live events and driving it around i made 109 mastery points
besides: with my setup it makes a nice street racing car and it is a force to be reckoned with at each live forzathon event.
use this car to unlock all streets too. you combine the effort there with making a ton of skillpoints