3. Buy gas on Wednesday: The theory is that if you buy gas on Wednesday, prices have come down from the weekend, when many gas stations elevate their prices in hopes of making a bit more profit from more people traveling. While a general rule would be to avoid buying on weekends, there are too many factors that come into play to rely on the Wednesday strategy. You are much better off looking at what gas stations are charging in your area through sites like AAA's fuel price check, GasBuddy or Gas Price Watch to find the best price on a daily basis.
4. Idle away: At one time, this may have been true, but with modern fuel-injection technology, it's no longer the case. Just as there is no longer a need to spend 10 minutes warming up your car on a cold morning, if you are going to be sitting for more than a half minute, you will save more gas by turning the engine off than letting it continue to run.
5. Sticker stats: The rating that automakers provide for gas mileage should be taken with a grain of salt. Beginning with 2008 vehicle models, the Environmental Protection Agency required fuel efficiency standards to be reported based on new test methods that better reflect actual driving conditions. Older models, however, may have derived their ratings via outdated or inaccurate methods.
While differences in manufacturer-devised MPG ratings will indicate one car is more fuel efficient than another, that is about all you can deduct from this number. You are better off looking at real-world mileage tests from Edmunds and Consumer Reports if you want a good idea of what gas mileage your car will really get.