Often, people who think they are being patriotic are actually just being statist/nationalist. In terms of America, being statist/nationalist is actually the opposite of patriotic.
Owning a gas-guzzler would be unpatriotic in my view, because our dependence on oil marries us to the Middle East. Having an American flag on your car can be patriotic or not, depending on your reason. If your reason is "I am proud to be an American", that is patriotic. If your reason is "I support my government regardless of what they do", that is unpatriotic. Having a Bush/Cheney sticker is incredibly unpatriotic because those guys shredded the Constitution.