Be wary of prepaid gasoline plans. Always fill the tank yourself before returning the vehicle so that you're only paying for the amount of gas you actually used. Try to avoid the gas stations right near the airport where you're dropping off your car -- the prices tend to be highest there.Even better: Check GasBuddy.com before your trip to find out where the cheapest gas stations are in your area. It may seem counterintuitive, but returning your car early may actually cost you money. You'll often pay an early return fee (usually about $15 a day), but even worse, your rate structure will most likely change and you'll be responsible for the difference. Of course, returning the car late could cost you too -- many car rental companies only give you a 30-minute grace period before beginning to rack up the late fees.