I decided a couple of days ago that I had too many cars in my garage which haven't been used so starting with Abarth I'm going through all my cars, finding a tune and design, taking it for a 2-3 lap (depending on class) race on Nurburgring Nordschleife + GP and then use the 2nd or 3rd lap as a hot lap, if the tune meets my requirements then I move on to the next car.
So far I've just reached the Ariel Atom so it will be a lengthy process but it's already giving me more choice when I'm in multiplayer lobbies. The AMC Javelin has been the big surprise so far, managing to grab 90th in C class on a messy lap.