Can diesel be this bad around me?

I'm from Green Bay, WI and this past week I took my 2001 7.3 crew cab f250 on a road trip to Ann Arbor, MI (about 500 miles). I usually average mid 14 mpg around Green Bay. While pulling a 6x12 fully enclosed u haul to Ann Arbor, I got about 13.5-14 mpg with Illinois diesel, which surprised me. But on the way home, with MI diesel from a speedway station, I got just over 17.5mpg (no u haul) doing a solid 75-78mph. A 3 mpg difference seems quite extreme but it was filled up to the very top and hand calculated.

My truck also has a 4" lift and 35s so I never expected over 16 ever. Any idea how that can happen?