The Speed Bomb is still the easiest way to earn lots of money, but you'll need to be AFK for long period of time in able to earn more money.

You want more money? Then here it goes! These are the guides to be followed:

1. Pick a vehicle you like, but better the fastest car you have so you'll have more than 100 km/h.
   
2. Choose Speed Bomb, of course.
3. You can choose any map you like, and choose peaceful mode because you'll AFK for few hours (if you want to).
4. Before you run (or while you're on the run), have a heavy object which will press harder and place to W key (which is forward/throttle) in order to press for long.
   
5. You can leave your computer for few hours. If you want to stop, then you can remove the heavy object and slow down in order the bomb to explode. :D
   
   I forgot to screenshot the last game about pressing brake, but this is you're going to do if you want to stop the Speed Bomb.

Welp, after just 20 mins or so, here is the result:

So if you want to have more money, then go for hours or so! :D

Also, this is after 3 hours of grind... got almost 600k!

Well, if you want more money (but no AFK), then go to easy to hard, then stick to some cars to get more money because the closer you get in cars, the more money you'll earn.
Moreover, don't get AFK, and don't get crashed on other cars. :P If you got fast speed and bumped to other cars, you'll instantly crash, so be careful on your speed.