I have 1000 miles on my civic my first 2 tanks were about 25 mpg then on my third I had 700 miles on it and i got 30mpg now on my fourth tank 1000 miles it dropped back to 26mpg. all of my driving is simular about half city half highway nothing really different on any givin tank is this normal? is my car not truly broke in yet and should I expect to see this? when it shot up to 30 I thought the rest of my tanks should only get better.