It sounds like your mom is threatening to "get mad" (with whatever that entails) as a check for keeping you under her thumb - you have to do EVERYTHING she says. Legally, she's right, but not for long. How soon do you turn 18?
1) How much money do you have in your own name, that she can't touch? If you have a joint bank account, be prepared to go to the bank on your 18th birthday, withdraw all the money, and open a new account in your name only. You do NOT want her to have access to your finances.
2) Is the car's title in your name, or your mom's? If it's still in your mom's name, ask her to transfer it into your name. If she refuses (which she probably will), start researching local legal options - you WILL have to take her to court over it (again, when you turn 18.) You will probably have to explain how you paid for the car at 14 in exchange for ownership of the car, how your mother has been using it without your permission, and how she refused to give you the title. With luck, you will end up the full and legal owner of the car, and you can either keep it (and ALL copies of the keys), or sell it and get another one. Be prepared to pay for your own insurance, which doesn't list your mother as a driver. And be ready to stand firm when she comes up with a bogus reason she should still have control over your car even when you're in college.
3) Is your mom paying for your college, or does she have to sign financial aid papers? My college roommate's mother refused to sign any of her financial aid paperwork unless she would come visit and do various things - completely trying to control her and her father (her parents were divorced). She ended up going to the school financial aid office and getting advice there. I suggest talking to your college's financial department sometime between now and your 18th birthday, to find out how you would handle being a completely independent 18-year-old. This may affect how you decide to deal with your mom.
Basically, if your mom is threatening to blow up over something as small as not leaving gas in the tank so she can borrow your car without permission, she will ALWAYS be able to control you unless you can call her bluff. It may be hard, and it may take some help from your older siblings, but you should be ready to kick her out of your life if you had to. Just knowing that you can do this will give you a lot more confidence. Additionally, when your mom learns you're serious and will kick her to the curb if she keeps pulling this crap, she'll have to decide whether her little power trip is worth losing you. She will probably always be somewhat manipulative, but when YOU are in control of your relationship (and can sever it if you want to), she will have to be more respectful of your rights and boundaries.