We also changed our to a wix filter as well and have just now run about a full tank out. I thought that it was solely the colder temps (and thus less A/C use) but now that you mention it, it might also be the wix filter. I got mine online from rockauto for $10.49 plus shipping but it and the cabing filter and shipping was $29.90. However, we got the ATP cabin filter (as it has charcoal which helps filter the cabin air better). It might have added 1/2 mile per gallon?