Smuggling
Most money in the game gets earned by smuggling booze & narcotics, this is a job which requires a lot of training or else you spend more time in jail than you see your own mother.
It is recommended to begin trying this from the very start of the game.
How do you do that?
Go to the “Smuggling” page (crime on the menu), on this page you see how many units you can carry. Select this many units of booze/narcotics with the lowest price and select buy, type in the code and press on the Buy/Sell button.
Repeat this till you succeed in buying the units.
Then “Travel” to a nearby town where you can sell the booze or narcotics for more money – and sell your stash at a profit!
Repeat this while you are doing crimes and stealing cars.
Remember: You will only win the rank-progress from buying booze and narcs once per hour, make sure you buy the maximum units allowed so you do not loose rank progress.