Will a bike fit in my SUV?
Generally speaking, if you drive a mid-size sedan or a SUV, you can likely fit the mountain bike inside with some work and possibly taking the wheels off. I describe this in Method #4 below if it works for you. I recommend putting a tarp down first to keep grease and mud out of your vehicle.
Do I really need a bike rack?
A proper bicycle rack is worth it even if only for the peace of mind they provide. Since most bicycle racks are also very affordable there is no need to put your equipment at risk in order to save some money. In the long run a rack will simply work out better for both your bike and vehicle.