With 30+ years of experience, I must strongly, but respectfully, disagree with Skid. Start out on a lower displacement as appropriate for your weight, beginner bike, learn how to control, brake, steer, maintain, etc... Then move up to a larger displacement as appropriate for your riding style.

There are differences between between types of bikes having the same displacement. Such as a cruiser or a crotch rocket. Both may have an equal displacement, but the characteristics and purposes are much different.

WEAR A HELMET! As others have said, take a MSF class, or other basic riding class. That way you have someone that may be able to keep you from making a few of the basic mistakes.

As you mature, it really isn't about the speed; It's about getting there in one piece.

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Remember, it's not just a car. It's an Adventure.