I'm not renting cars now, but when I did, I would just find the cheapest middle-tier car I could to reserve. The website always had, for example, a Chrysler 200 and a BMW in the "luxury" tier, the former for half the price, cus it's a POS. But, in my experience, no matter which one you reserve, you're gonna get a random vehicle from the tier (or higher), so might as well save the money.