I'm going to suggest what I suggested to a coworker awhile back: figure out the total monthly cost of buying the car. Include insurance, gas, mortgage, tolls, etc. Try to be as realistic as possible with the numbers. This last point is actually really important, especially at your age - when I was 17, I didn't have a clear idea of how much those things actually cost. Hint: it's a lot more than you think. Even if you think you know how much those things cost, it still costs more than you think. Again, it's really important that you include the actual total cost of car ownership - if you can afford the payment and insurance but it leaves you without money for gas, your car won't be going anywhere.

Don't forget, too, to include saving money for routine maintenance like oil changes. In a 9 month period, you will likely need 1 - 3 oil changes, depending on how much you drive.

Once you figure out the actual total cost, start saving that amount every month without fail - as in, move it out of checking and into a savings account on the same day every month and don't touch the money under any circumstance whatever. Do this for a minimum of 9 months (and preferably 12). If you come up short even one month for some reason, "dip into" the money in savings, or find that you can't keep up with it, keep in mind that that would've been your car payment you missed had you actually gotten the loan. If you find that

You don't really mind not being able to spend all that money

You made all of the "payments" on time

You never dipped into the savings

then you have evidence that you'll be able to make the payments. And, as an added bonus, you'll now have more money for a down payment.

Note that the "9 months" part is important. Yes, this means waiting until after you turn 18, but as the other answers have indicated, it's not feasible to get it that soon anyway no matter what you do. Besides, if you really want it as much as you say you do, you'll presumably still want it in 9 months.

One more thing I'd like to point to: the Stanford marshmallow experiment. In this study, preschool children were given a marshmallow (or a cookie) and were told that if they still had it when the researcher returned in 15 minutes they'd give them another one. The children who had a second one ended up being vastly more successful later in life - better education, more healthy higher SAT scores, etc. In fact, whether they ate the cookie before the researcher returned evidently predicted their BMI 30 years later. Point being that delayed gratification is one of the most important skills to learn. With that, to turn to the (now-edited-out) statement:

I desperately need this one...

Some of the other answers have touched on this, but I'd recommend distinguishing between a "want" and a "need." Think objectively about the actual consequences of not having this particular car. Does not having it mean that you'll die? Will you not be able to live a happy, fulfilled life if you don't have that particular car?

The first thing to understand is the role of your beliefs here. As the Roman philosopher Epictetus said in his Enchiridion:

Men are disturbed, not by things, but by the principles and notions which they form concerning things.

People don't react to things or events - they react to their beliefs about the things and events. Albert Ellis taught the "ABCs" of how we actually react to things:

Activating Event -> Beliefs (your interpretation of the event) -> Consequences (your reaction)

Everyone approaches life with a set of expectations and preferences. These can be broadly categorized into expectations and preferences about ourselves, expectations and preferences about others, and expectations and preferences about general world conditions. There's nothing wrong with having expectations and preferences - the problem is when you demand that they always be met. This results in people overreacting when things don't go their way. And guess what? It won't. People will do things you don't like all the time. You will be treated badly at times, and people will let you down. World conditions can be terribly unfair at times. You'll even let yourself down at times.

While we have a lot of power to improve our circumstances, there's actually a lot of stuff that we have no control over whatever. Again quoting Epictetus:

Some things are in our control and others not. Things in our control are opinion, pursuit, desire, aversion, and, in a word, whatever are our own actions. Things not in our control are body, property, reputation, command, and, in one word, whatever are not our own actions.

In other words, ultimately, the only thing you can control is yourself. Point being that your preferences and expectations will be violated eventually - in fact, they'll probably be violated quite often, and often in ways that you can't prevent or control in the least.

Going back to the issue of going from a preference/expectation to an inflexible, rigid demands, you go from "I really want this car" to "I absolutely must have it - I need it." The first statement isn't a problem, but the second statement is.

I'd strongly recommend reading A Guide to Rational Living by Albert Ellis for a lot more on this point.

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