When BMW released the new 3-Series to the press, one of the most remarkable points was the exceptional gas mileage the new 328i model received over the outgoing model.
The EPA may have felt the mileage might be a bit too remarkable and ran the 328i through the paces to see for themselves. The end result was a 3 MPG difference, which doesn't sound like much in the real world, but in marketing terms it is pretty big deal. BMW currently is at a loss to explain the difference and has issued the following statement on the discrepancy.
BMW Press Release:
All manufacturers are responsible for testing and submitting fuel economy figures each year to the EPA (based on physical tests following the EPA test cycle on a dynamometer). Occasionally, the EPA will validate the figures that we provide. When this happens, there are sometimes small changes in the published EPA fuel economy figures (usually 1 mpg up or down).
The EPA recently tested the 2012 328i Automatic and obtained a highway mileage result that was 3m pg lower (33mpg versus 36mpg) than the BMW test result. We are very surprised by this result and are currently working to determine how this is possible. The new rating seems abnormally low in relation to the other models in the BMW range and by the real-world fuel economy that we are seeing from this model.
Unfortunately, there is no provision in the EPA rules for a re-test this year. Therefore regardless of the results of our investigation, the new rating will stand for this model year.