User blog comment:Sirebel/Gold, Gold and More Gold/@comment-86.163.140.123-20160615205111/@comment-27712397-20160616195841

Go to a series you've completed, especially the early ones. I found that if I selected a time trial, the game would load the leaderboard, but decide that I didn't have a time recorded, and would mark the event as a new one. At that stage it said that series completion was still 100%. But if I then closed the app without doing the time trial, when I restarted the game the series would now only be 98%, and doing the time trial gave me a repeat completion bonus. Repeat for the next TT in the series for another bonus, and so on. All perfectly legit because there's no 'borrowing' of gold or manipulation with cloud saves, multiple devices or anything like that.

Plus you get to drive all the cars you forgot you even had!