To get the fastest car called the pagani huayra all you do is beat the run. I've done it like 150 times sooooo, yeah it's pretty easy.
Lamborghini aventador
All though I havent gotten it yet in know that it is in there the lamborghini aventador is an exclusive in the game nfs the run (3ds)all you need to do is get to the last rank (which is 20 or 21 it is called legend. The fastest way to get it is to play challenges, loganboyee.