You basically get to pick from whatever is available at the moment you are at the counter. I always book Economy or Compact. I virtually never get what I book.

I figure the 1/8 or so of rentals where I get a Hybrid that uses a lot less gas makes up for the 1/5 of visits where I get upgraded to an SUV. I'll wait before I'll take a truck or van.

Also these comments are consistent with all agencies at airports, not just Avis/Budget. Enterprise used to be really good at finding a 40k mile beat up compact but the past two years even with them it is back to the old whatever is there at the moment thing.