Well to be honest I don't think this is such a good idea. Since you want to run away from home without telling your mom what are you going to do when you run away?
Do you have money for necessities? Such as food, gas for the car, a place to live. Can you even pay for school? There are many things you have to think about before moving out. I'm currently working and I still live at home because the cost of moving out is too much.
Plus I'm pretty sure you really can't do much since the car will most likely be under your parents name so if they wanted they could take the car back. I'm sure they could just call the cops and eventually they'll find you. Technically you would be stealing their property.