I invested in a $20 5 gallon gas can in September to capture all 20 gallons for each fill up, and generally use it to fill up a small tank car (Prius) with the excess from our other 2 cars. On long car trips where I don’t bring the gas can, I’ve found it’s possible to add about 10% of tank capacity once the gas pump clicks off. In rental cars, I just keep filling until there’s gas in the fill neck of the tank