What are the coins for in street gears

Im not sure either I have been wondering the same thing for months now I have well over 999 Gold coins. The best answer Ive got so far is by asking another player but he spoke German! no surprise but from what I could make out from his English was that the gold coins were for races? so i would expect them to be a type of pay for special events or races. Other than that I have left all my quests uncollected if they are gold coins. Hope this helps, you can try to do the same and ask online! =)