I recently started a new game by deleting my savegame.
What I do now:
Everytime I get a multiple of 30 cars I recieve a paint as a reward.
If it is not a chrome, I do not click "ok" but hit the PS3 button and end the game.
Then restart GT5, the window with "you have xx cars in your garage" pops up again, with a new paint chip as a reward.
I repeat this process untill I get a chrome.
Sometimes it taks 2 times, sometimes 5, but never that much.
I now have 91 cars and 2 chromes, so it worked twice (forgot to do it at 30 cars).
Easiest way to get gold or silver chrome.
On my old savegame I had 1031 cars and only 1 silver chrome paint chip, which was awarded after winning the Kart challenge.
