I just stole my stepdads car to go get chicken nuggets
You couldn't really have gone for nuggets, have you?
Chicken nuggets are really good. I'm really hungry because I didn't eat breakfast
It is never a good idea to steal someone's car. It does not matter the reason, it is illegal to steal a car.
I'm only borrowing the car, so there.
Even though you thought it was borrowing, your stepdad wasn't aware of it and so you were stealing it. What if he found out midway, he might have thought a criminal stole it and caused a scene. I think it's wise to apologize to him and accept whatever punishment that will come.