I thought you only could expect to do this if you'd driven the car under a certain number of miles
fewer than 50, maybe? I remember when we rented a car in Pittsburgh. The tank was FULL when we left the airport. I drove to the hotel (near the baseball stadium) and we only drove the car across to the other side of the river once all weekend, before returning it to the airport. I missed the last gas station, but the needle on the gauge was just at the bottom of the red line for F (hadn't used much gas at all) and so I just returned the car. No questions asked. On a subsequent rental at a different airport, they mentioned that if we returned the car with a very few number of miles driven, we'd be required to show a gas receipt, so I couldn't get away with "free gas" like I did in Pittsburgh!