Asphalt Overdrive

After beating the turf 4 boss and getting the Lamborghini Countach, DO NOT waste money on upgrading it. It has a max power of 1586 and the final boss requires a car with a power of 1660. Therefore, simply grind until you get 750k so that you can buy the Lamborghini Veneno. This car goes up to 1726, leaving you with plenty of room to upgrade your car.

I wasted so much money upgrading the Countach until about 1540, only to realise I had to buy a new car. Trust me, it was painful.. So, do the right thing, and buy the Veneno immediately.

Hi everybody,

(Edit: This is a solution to people who don't want to spend real money!)

This solution is to explain how to not waste too much time and credits.

Once you beat the turf 4 boss, try to beat it again to win his car, the Lamborghini Countach (this will cost you 35 gold, or 40 to play the second boss race). Do not upgrade this car. Save your money and earn the maximum stars you can in turf 5 with it. Once you get 750 000 credits (you get 500 000 by beating turf 4 boss the first time), buy the Lamborghini Veneno.
Winning 3 stars in a race will accord you an average of 50 000 credits (in turf 5). Replay it will accord you about 20 000 credits, depending on your skills.

(If you're good enough, you can try some event wich is a good way to earn credits and/or gold.)
((You can aslo replay races from any turf you want if you're bored playing the same race the whole time, but you'll get less credits.))

Now you gonna get stuck (or about to get) by the power limit of your car and don't have credits to upgrade it, do the Veneno "forbidden race" (this is the french translation sorry) top left of the screen. It will unlock after you play the Countach "forbidden race".
This is a fast race where you make an average of 23 000 credits when finnishing it.

Now you have to grind about 1 500 000 - 1 750 000 credits to
- Upgrade motor to level 4 this way you'll have the amount power required (1641) to play the final "escape" (french translation) mission. Achievement is yours if you get at least 1 star.
-Upgrade the nitro to level 2 to get your car 1661 power which is the open door to race against the turf 5 boss. Achievement is yours if you beat it.

I hope this solution will help some of you.
Sorry for my bad english too.

Cheers.