My rule of thumb is to never return the rental car with less gas than was in the car when first rented. The rental agency will charge you at least .30 or more than the going gas rate. This may not seem like much, but add up the price per gallon and the number of gallons they may put in the tank, and it may be significant. Just make a quick stop at a gas station outside the rental agency right before you return the car, and you'll save a few dollars.