What do you think about me choosing the car that my mum buys for me?
Why do you want to choose the car?
Because I want it to be a cool car.
It's terribly rude to try and dictate which gift someone gets for you. You should be more grateful for such an expensive present.
I'm sorry but I'm very picky and I want it to be something that I like.
I understand that you want to like the car, but if the car is a gift from your mom you should be happy no matter what she buys for you. I think it would be impolite to tell her that you only want one specific car.