How to convince my parents to let me get a dirt bike?

I've wanted a dirt bike since I was a little kid. I think I've asked about 1000 times. I'm 18 years old and I have enough money to buy one and my dad said to save the money for college even though I have enough, keeps saying we have no where to put it but we have a 2 car garage (no cars are ever in the garage) and a small storage space in the back yard, and also he said he would rather have an ATV and my dad has only rode an ATV once and he's NOT the one buying a dirt bike. What should I do? Im thinking to just buy it anyway.