I was getting really bad mileage and also having some other issues getting worse, i changed up my air filter to a k&n high flow air filter (which is also reusable for the life of the vehicle, just need to clean it every 3 months or so!) and noticed a CONSIDERABLE improvement in gas mileage and some issues went away completely(including a knock in the engine!), try changing your air filter and see if it helps (normally supposed to change it every 24, 000 km anyways)