Originally Posted by Wildsailor
I am not entirely sure but what made me think of it was the idea that the SFE package, which comes with wheel covers that block off the air passage through the wheels, is worth 2 MPG highway. The 18" package is wider and may add to the frontal area. If the Focus is that sensitive that a wheel cover can make 2 MPG difference then wider tires and wheels may have an impact.
Two cars side by side, one with 16" and one with 18", doing a coastdown would verify.
In the meantime if those that are struggling to get to the numbers being posted for FE are all 18" Sport Package Titanium then that would start to fill in a pattern.
Remember the Cruze Eco has big ole' 17" multispoke alloys, similar to our 17" wheels. So I think the solid wheel covers of the SFE are more of a marketing gimmick than any real savings provider. It has more to do with weight reduction; the steel wheels and plastic hubcaps weigh less than the alloys.
Also, if the Ti's 18" wheel option resulted in such a significant mileage penalty I would think the EPA would require them to test and rate them separately. But maybe I'm naive...