I have calculated the mileage for my "S" based on 18 fill ups since I bought the car new at the end of September. I used the average speed as calculated by the trip computer on the car, the MPG shown on the trip computer and the actual MPG calculated at each fill up. I reset the trip computer after every fuel stop. My average speed has been 42 mph with MPG calculated by the car of 20.56 MPG. The actual calculated mileage was 18.78 MPG. This variation of between 1.5 and 1.75 MPG has been consistent every time I have filled the car. It would be interesting if other are seeing this variation.