Be On the Lookout For: Rental Car Gas Prices The potential cost of a car rental: $325.15, and that would only cover the gas. This is the new reality for those who need to rent a car. A new USA Today survey shows some rental companies are now charging over $9 a gallon to refuel returns when the customer neglects to fill up. The paper surveyed rental companies at 13 airports nationwide and found that Hertz charges $9.29 a gallon. Dollar and Thrifty charge $8.99 at some locations. Renters can avoid the high fee by prepaying for a full tank of gas or simply filling up themselves at a local gas station. The prepay price is cheaper. It's closer to the average price at local gas stations, sometimes even less than, but there is a catch. Renters who are trying to save money by prepaying for a full tank should be sure to use up that whole tank; they won't get a monetary credit for the amount of gas left. So, be on the lookout for gas prices the next time you rent a car or you might end up paying more for gas than for the rental.