I consistently achieved 23 MPG last summer while I had the 245's (90% highway). I changed from stock steelies and 245's to TRD Trail Teams alloys and 265's in November I think... and noticed about a 2 MPG drop to 21 MPG... but I am still not convinced this is accurate. I think there may be a combination of factors involved that make my particular data incorrect...
Speedometer inaccurate with 245's anyway?
Winter blend gas?
Tire size change?
I have ruled out the wheel change as a factor because I have since changed back to my stock steelies with the 265... nothing changed with my MPG when changing from alloys to steelies.
I also don't really see the tire change as a big deal as the 2 tires sizes being compared have very near the same revolutions per mile... see data below from this tool:
Hopefully when the summer blend gas arrives I will know for sure if that is even a factor. I would like nothing more than to return to my 23 MPG numbers with my 265's.