BMW confirmed to us today that the EPA’s fuel economy estimates for the 2012 328i automatic will be revised, dropping from 24/36 mpg city/highway to 23/33 mpg—a difference of 1 mpg in the city, and 3 mpg on the highway. As the EPA’s rules don’t allow for a retest, the revised rating will stand for the 2012 model year.

BMW says 328i models equipped with the eight-speed automatic transmission are the only models to receive a revised estimate, with the six-speed-manual-equipped version retaining its 23/34-mpg rating. BMW is investigating the reason behind the discrepancy, but regardless of the automaker’s findings, the EPA’s estimates will remain for the 2012 model year. The government’s fuel economy website currently doesn’t show the change, however BMW’s consumer page has been updated to reflect the revised numbers.

A BMW spokesperson told us that the EPA doesn’t test every car, and manufacturers must instead conduct their own tests following the EPA’s test cycle and submit figures each year. Sometimes, the EPA will validate those figures by repeating the test cycle itself. In this case, the EPA requested a 328i automatic for testing, and came up with different numbers.

We’ve been impressed with the performance from the 2012 BMW 328i’s new turbocharged four-cylinder thus far, and the promise of better fuel economy was a have-your-cake-and-eat-it-too kind of proposition. Now, we suppose you just get slightly less cake.