There shouldn't be any surprise that hanging anything off of a car is going to add a little bit of additional aerodynamic drag, and that translates to a slight decrease in fuel economy. For one driver, the change is practically unnoticeable, but for a legion of fans, it can add up.
Treehugger, a website which clearly wears its pro-environment slant on its sleeve, cites a study by Dr. Antonio Filippone of Manchester University of the 2006 World Cup. It found that a car with two small flags attached used about a quarter gallon (one liter) of additional fuel per hour when traveling at 70 miles per hour. Referencing vehicles in England at that time, the study suggested the waving banners could account for a three-percent loss in fuel economy. However, Dr. Filippone estimates that with 500,000 cars flying the colors in England during that year's World Cup, the flags added an extra 322,000 gallons of gas and 6.2 million pounds of CO2, according to the BBC News.
Given the English teams' lackluster performance in this year's World Cup, there probably aren't too many flags flying in Blighty this year, but there are doubtlessly hundreds of thousands of similar flags proudly flying on cars in World-Cup-crazy countries around the world. No one is saying that a three-percent change in economy means that you shouldn't support your team, but it's interesting to contemplate how these tiny changes can add up.