how long have you had it and if more than 5000 miles then it could be that tires are over or under inflated or you may not be maintaining the car every 3000 miles . you could be continuously resetting the mpg indicator if you continue to reset it you will never know what you get it may show you what you got for a certain trip or something but you will have to let it continue to calculate for at least 4500 miles instead of resetting it, and check those other things, keep it on a full tank, and you will see a difference
