The spike in fuel costs is driving a big demand for fuel-efficient car rentals. Too big actually. Car rental companies typically offer something for everyone, from the compact Ford Focus right on up to a full size Toyota Avalon (pictured left). Problem is, too many people, it seems, are reserving the smaller, fuel efficient drives only to find they've been (involuntarily) upgraded when arriving to pick up the vehicle.This article from the L.A times uncovers what car rental contracts say about these less than desired upgrades, and how reserving a compact car may actually start costing you more.