my wife's car says it should be 24 MPG in the city and it only got 18 mpg on this last tank full, (320 miles from 17 gallons) .. has anyone else noticed their cars don't run as efficiently as the mpg from gas quality or such or could their be an issue that I need to go to a specialist to get checked.. from my calculation she is missing at least 100 miles from a full tank.. thanks guys.