Pour in the additive: According to the EPA, fuel additives are a waste of money that don't contribute anything to mileage.
Change that dirty air filter: Changing your air filter might make your engine last longer, but it won't help gas mileage, according to a test done by Consumer Reports. It seems computerized fuel controls take a clogged air filter into account so that it doesn't cut into fuel consumption.
Fill up when it's cold outside because gasoline is denser: The gas is stored underground in huge tanks and the outside temperature has little effect on it.
Pump up those tires: Under inflation means less (4% less) mileage, so overinflating must add mileage, right? Nope. Not only does it not improve mileage, it can lead to accidents as overinflated tires have less grip. There's a reason it's called the “recommended” inflation pressure.
If you're only going to wait a minute or two, keep the engine running: Drivers once firmly believed that it took much more gas to start an engine than to keep it idling. And it might've been true. But fuel injected engines do not do not waste gas when starting. Idling can use up about a half gallon per hour, something to keep in mind during traffic jams.
To really save gas: Stop driving like a jerk: Not surprisingly, studies show that jack-rabbit starts, accelerating up to stop signs and red lights, tailgating, and serial lane changing can cut fuel mileage by up to 50%.