That's the point of reserving a car class. The guarantee isn't perfect: if a few renters keep their vehicles beyond the reserved return date it's possible some renters will be pushed into other car classes. Bargain for a free upgrade into a 3-row luxury SUV at that point. Your chance of getting a minivan if you have a minivan reserved is much better than somebody hoping to pay a midsize rate and take one from the Emerald Aisle.