I have a 2016 civic lx 2.0. I drive 120 miles daily . 98 percent highway. No stops 65 mph cruise on the whole way. Fuel mileage was always 43-44 mpg after break in. Getting 38-39 now. It does have a 150 thousand miles on it. New air filter, always keep oil changed when needed. Is this decline in mileage normal for the miles on the vehicle or could it be something else. Curious if anyone has ideas or seen this happen on their civic.