It is for a number of reasons. First of all, your insurance will be higher than usually until you turn 26. Many rental agencies won't even let you rent a car until this age due to the insurance issues. The fact that you got a new car that is also a sports car is not helping you as those have a higher collision rate. The fact that you live in Miami, which is a very urban area with lots of cars and accident rates also makes your rate much higher than usual. An older used car with four doors that is not considered a sports vehicle will garner you a cheaper insurane rate. If you don't get full coverage, that will also be considerably lower.
If you use your grandfather's insurance, this creates an issue because if you get into an accident, you may not be covered depending on the situation, especially if they find out that your grandfather is not the primary driver of the vehicle.
Hummers have high insurance rates because they are considered luxury vehicles and you're still 22 so I wouldn't suggest a Hummer. A Honda Civic should have a normal rate. What you can do is actually call your insurance agent and ask him to price some insurance rates for different cars for you and then you can decide what you want.
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