Ways to convince my dad to get me a mustang?

Okay this is the story I got a 85 bronco but gets 12 mpg and it breaks down all the time im finishing up fixing it again for the road now and I asked my dad for a mustang this month for my graduation and 18th birthday present and he said okay but hes waiting on these loan people to call? but im just worried that he'll change his mind and say well you just fixed your truck so you don't need a car, and I need something to say back when he says that.