John Fischer, owner of a consulting firm in Houston, booked a midsize rental car with Avis for his trip to Orlando last month.

So he was surprised when the agent gave him a free -- but unwanted -- upgrade to a Hummer H2. "My gas bill was the same as my rental car bill for the four-day rental," he says.

In a reflection of how rising fuel prices are affecting all aspects of travel, demand for small and midsize rental cars is rising rapidly even as rental companies struggle to keep them on their lots. The shortage means rental companies must steer customers to larger vehicles, usually waiving the upgrade fees -- but forcing consumers to pay more in gas, according to USA Today.