It is claimed by the company NitroFill and the GetNitrogen Institute that filling car tires with nitrogen improves gas mileage considerably. The reason given is that oxygen leaks out of tires so that the increased rolling friction causes a reduced gas mileage. Because it is hard to do an actual road test, we report on a simple visual test of leakage from oxygen- and nitrogen-filled balloons. This experiment can be repeated in classrooms to address a controversial and topical issue, while at the same time highlighting hypothesis formulation, verification, and falsification in scientific experiments.