Your chances of getting a car (including possibly an "upgraded" car) increase if you sign up for the free Avis Preferred program. That way, when you get to the airport, the car you're assigned should be shown on the Preferred board and you can go right to it. However, the car you get assigned might be an "upgrade", so it depends if you like that or not. On my last Avis Preferred rental, at ABQ, I reserved an Intermediate (such as a Toyota Corolla), because it was the cheapest category several months out, and when I got there they had assigned me a Toyota RAV4 high-clearance SUV, which was great, because I actually had to drive -- and park in pullouts -- on an an unpaved sandy road in White Sands National Park and on a fairly bumpy unpaved road at Ghost Ranch, and I felt much more comfortable with that kind of vehicle on those kind of roads.