I don't know what it could be. I had 3.55 in it a few years ago, but didn't care at the time what the mileage could be. I still don't think it ever got above 12-15 mpg. I rarely drive the car in OD unless I'm on a long stretch of highway becasue with the 2.73 in 3rd its not turning no more than 2400 at 70 with a 25 in. tall tire.