My parents won't let me get my own car, what do I do?

I am an 18 years old guy, who goes to school and has a part time job. I recently got my driving license, and I've been wanting to get my own car, however my parents won't let me get one.

Why? Because we already have 2 cars. My dad's car, and my sister's car. My dad says: "But we already have 2 cars, if you need a car, just take our keys and go for it."

My problem is, I want my own car. Mine. Sometimes my dad can be at work, and my sister can be at university, and I could be wanting to go somewhere, and I'd want a car (public transportation is available but I never liked it), so that I can move wherever I want, whenever I want.

The car's costs are not an issue, I can cover gas, insurance, maintenance, everything. He just won't let me get my own, because his and my sister's are available.

How do I convince him to let me buy it?