**THIS SOLUTION NOT LONGER WORKS FOR THE NEW VERSION OF THE GAME, feel free to check other solutions or comments from this guide***
I really don't know if my strategy was the most efficient but anyway...
I dominated all chapters, every single one, to earn enough coins, credits and kits.
What I did first was spend coins till I had four cars of 3 stars o more. I ended up with one 3 starts Sport, one 3 starts Muscle (this was from the event), one 4 stars Street and one 4 stars Super. After that, I spent all my coins in credits, several times I wanted to get more cars but I only got duplicated ones. So... spend coins in credits to level up is more guaranteed that spend them in cars.
For the kits to upgrade cars, in the first and second episodes I always race against the 300-points-cars to earn more cards to use to get more kits. In the third episode it was not necessary earn more cards since by dominating chapters the game gives you level 3, 4 and sometimes 5 kits to upgrade.
I first upgraded my 3 and 4 stars cars, after that I upgraded 2 stars cars. I also reclaim the daily car card to have enough to upgrade.
DO NOT SPEND COINS IN ENERGY NOR KITS TO REPAIR. ONLY IN CAR CARDS AND CREDITS.
Take that recharge time to rest after playing the same races over and over again.
Hope this help.