Pretty simple. Just pick your favorite vehicle and constantly race with it, replacing your loot with better performance loot as you go along.
However, for those of you who just want to 'speed' through it, I went with the Suzuki Motorcross
Bike (All available vehicles for the Motorcross should do fine, so feel free to choose whichever you already have the highest performance value for).
Track: White Mountain
Video for a route to take:https://www.twitch.tv/videos/281554999