I get this question frequently when the seasons change. Here in Sacramento California when summer approaches,the fuel manufactures add additional chemicals to gas to compensate for hot weather and emissions. This can decrease mileage by 1-3 mpg. Don't be alarmed as this is normal.
Another issue could be the engine is over due for a tune up or is getting a buildup of carbon deposits from using cheap gas that does not have cleaners added. Getting a de-carb procedure performed every 30k miles is recommend. Also choose gas that advertises cleaner has been added. It's usually more expensive. You can also add fuel additive to your gas tank.
Last but not least if you're still concerned schedule a visit to your trusted repair shop for a check up.