How or what is the best way to earn credits as I am after the f1 car… N I’m 800.000 short. 
??
Keep driving! Turn off assists.
Do Rivals with a well-known car and get a good setup for it. By storming the leaderboards, you should be able to make, let’s say, 200K per hour (from driving credits and level-up credits).
Just play the game man. The credits start to stack up quickly. I don’t play much and I have $73 million with nothing to spend it on.
The fastest way is to check your Forza Rewards. You might have $1 million waiting for you to claim. There’s a drop down box at the top of this website to claim them.
Or you can:
-Do rivals because you get bonuses for beating your rival.
-Focus on one affinity to get cash bonuses for leveling up.
-Turn assists off for greater multiplier
-Do more career races with one affinity with less assists
Just be glad the prices of cars are half of what they used to be. They were dropped because people were too impatatient to save up.
Now most have a kagillion credits with nothing to buy as result.