It is incredibly easy to arbitrage (I call it "carbitrage") rate classes.

When the gap between the rate of the smallest car you are willing to put up with and the car you want is too large, reserve the smaller car and negotiate at the counter. If they have inventory, it is usually far cheaper to upgrade then.

When reserving my last rental (Alamo), I had the choice of a mid-size for $16 or an SUV for $45 a day. I took the mid-size. At the counter, they offered to upgrade me to an SUV for $7 more a day. Bingo!