So, why do we need to use Bitcoin???
to buy a new car

LOL
maybe this is absurd reason but this is my reason just wanna buy car
That's foolish to think of using bitcoin to buy a car. In future users might get a access to buy car using bitcoin, but as the above mate indicated using bitcoin to buy car is not possible. Better is to make a investment into bitcoin from the day job earning, which would help get a car with bitcoin in future as we can buy only digital goods and other few services for bitcoin.
Maybe you cannot buy a car directly right now but maybe we could do that in the future and if you have enough bitcoin and the seller is willing to accept bitcoin then you could buy a car right now, it is unlikely but you could try.