I don't remember the last time I put gas in a rental car before returning it. Sometimes, if it was a short trip, i don't get hit with a gas charge. The needle says "full" (or close enough) and the rental car agent calls it full. Sometimes I've driven the car for a day or two and I get hit for half a tank, maybe more. Well, I used it, why not pay for it? Yeah, it would be less expensive if I found a gas station and filled it myself. Guess what? Most gas stations near airports have higher prices for the gas at the pumps. Especially if it's right off the freeway at the exit just before the airport.

I used to be religious about filling up before returning the rental car - but then I had to find the right gas station. And if I didn't know my way around, I could easily see stopping late at night in a bad section of town. So, screw it, I return it as it - and pay the higher cost per gallon. 99.9998% of the time it's a business rental and I don't get too worked up about it. The times that it's a personal rental and I might be paying a bit more for gas? Convenience.