In Dr. Driving, your goal is to complete various missions, ranging from speeding like a maniac to drifting like a psycho to… conserving your gas and driving like an old lady! This oddball game has two forms of money: coins, which are your normal money, and gold, which is way more valuable. One is hard to earn, and one is really easy to earn, but both of them can be had for free.

Source: http://ciriya.com/apptricks/dr-drivi...and-free-gold/