Question... What mpg you getting on that tire you have? What brand and size?
They are Goodyear Duratracs in the SL (standard load) rating, 275/65/18. i'm getting about 18-19 MPG in my usual commute which is about 15 miles each way, half hwy and the other half city including downtown Salt Lake City traffic.
On longer hwy trips which usually include mountain passes and 80 MPH speed limits I get between 21-23 MPG. Also on longer trips I'm usually running pretty heavy full of camping gear/recovery gear/etc.
I hand calculate every tank and also account for the roughly 5% error from the tire size change. On a long stretch north of Flagstaff AZ running a pretty steady 65 MPH the EVIC was showing just under 30 MPG....that tank I ran from Flagstaff AZ to Taylorsville UT which was just few miles under 500 miles and I still had almost 2 gallons or so left in the tank if I remember correctly.