My preferred rental car is a Prius, but it is never worth it to me to reserve one directly -- the price is often $100+. So my technique is to rent in a cheaper class, and pray that my Presidential Club two class upgrade will qualify me for an upgrade. It almost always seems to come to down to desk agent discretion. I'd like to eliminate this crapshoot.

Does anyone know what class I must book so that a 2 class upgrade will qualify me for a Prius? Thanks!